diff --git a/substrate/core/sr-io/src/lib.rs b/substrate/core/sr-io/src/lib.rs
index 6721cc7ecd..f340f0a99b 100644
--- a/substrate/core/sr-io/src/lib.rs
+++ b/substrate/core/sr-io/src/lib.rs
@@ -394,7 +394,10 @@ mod imp {
 }
 
 #[cfg(feature = "std")]
-pub use self::imp::{StorageOverlay, ChildrenStorageOverlay, with_storage, with_externalities};
+pub use self::imp::{
+	StorageOverlay, ChildrenStorageOverlay, with_storage, with_storage_and_children,
+	with_externalities
+};
 #[cfg(not(feature = "std"))]
 pub use self::imp::ext::*;
 
diff --git a/substrate/core/sr-io/with_std.rs b/substrate/core/sr-io/with_std.rs
index 25be27046a..9475aa0715 100644
--- a/substrate/core/sr-io/with_std.rs
+++ b/substrate/core/sr-io/with_std.rs
@@ -435,9 +435,32 @@ pub type ChildrenStorageOverlay = HashMap<Vec<u8>, StorageOverlay>;
 pub fn with_storage<R, F: FnOnce() -> R>(storage: &mut StorageOverlay, f: F) -> R {
 	let mut alt_storage = Default::default();
 	rstd::mem::swap(&mut alt_storage, storage);
-	let mut ext: BasicExternalities = alt_storage.into();
+	let mut ext = BasicExternalities::new(alt_storage);
 	let r = ext::using(&mut ext, f);
-	*storage = ext.into();
+	*storage = ext.into_storages().0;
+	r
+}
+
+/// Execute the given closure with global functions available whose functionality routes into
+/// externalities that draw from and populate `storage` and `children_storage`.
+/// Forwards the value that the closure returns.
+pub fn with_storage_and_children<R, F: FnOnce() -> R>(
+	storage: &mut StorageOverlay,
+	children_storage: &mut ChildrenStorageOverlay,
+	f: F
+) -> R {
+	let mut alt_storage = Default::default();
+	let mut alt_children_storage = Default::default();
+	rstd::mem::swap(&mut alt_storage, storage);
+	rstd::mem::swap(&mut alt_children_storage, children_storage);
+
+	let mut ext = BasicExternalities::new_with_children(alt_storage, alt_children_storage);
+	let r = ext::using(&mut ext, f);
+
+	let storage_tuple = ext.into_storages();
+	*storage = storage_tuple.0;
+	*children_storage = storage_tuple.1;
+
 	r
 }
 
diff --git a/substrate/core/state-machine/src/basic.rs b/substrate/core/state-machine/src/basic.rs
index e9939711f1..e922db260c 100644
--- a/substrate/core/state-machine/src/basic.rs
+++ b/substrate/core/state-machine/src/basic.rs
@@ -18,67 +18,64 @@
 
 use std::collections::HashMap;
 use std::iter::FromIterator;
+use crate::backend::{Backend, InMemory};
 use hash_db::Hasher;
 use trie::trie_root;
 use primitives::offchain;
-use primitives::storage::well_known_keys::{CHANGES_TRIE_CONFIG, CODE, HEAP_PAGES};
+use primitives::storage::well_known_keys::{HEAP_PAGES, is_child_storage_key};
 use parity_codec::Encode;
-use super::{ChildStorageKey, Externalities, OverlayedChanges};
+use super::{ChildStorageKey, Externalities};
 use log::warn;
 
 /// Simple HashMap-based Externalities impl.
+#[derive(Debug)]
 pub struct BasicExternalities {
-	inner: HashMap<Vec<u8>, Vec<u8>>,
-	changes: OverlayedChanges,
-	code: Option<Vec<u8>>,
+	top: HashMap<Vec<u8>, Vec<u8>>,
+	children: HashMap<Vec<u8>, HashMap<Vec<u8>, Vec<u8>>>,
 }
 
 impl BasicExternalities {
 	/// Create a new instance of `BasicExternalities`
-	pub fn new(inner: HashMap<Vec<u8>, Vec<u8>>) -> Self {
-		Self::new_with_code(&[], inner)
+	pub fn new(top: HashMap<Vec<u8>, Vec<u8>>) -> Self {
+		Self::new_with_children(top, Default::default())
 	}
 
-	/// Create a new instance of `BasicExternalities`
-	pub fn new_with_code(code: &[u8], mut inner: HashMap<Vec<u8>, Vec<u8>>) -> Self {
-		let mut overlay = OverlayedChanges::default();
-		super::set_changes_trie_config(
-			&mut overlay,
-			inner.get(&CHANGES_TRIE_CONFIG.to_vec()).cloned(),
-			false,
-		).expect("changes trie configuration is correct in test env; qed");
-
-		inner.insert(HEAP_PAGES.to_vec(), 8u64.encode());
-
+	/// Create a new instance of `BasicExternalities` with children
+	pub fn new_with_children(
+		mut top: HashMap<Vec<u8>, Vec<u8>>,
+		children: HashMap<Vec<u8>, HashMap<Vec<u8>, Vec<u8>>>,
+	) -> Self {
+		top.insert(HEAP_PAGES.to_vec(), 8u64.encode());
 		BasicExternalities {
-			inner,
-			changes: overlay,
-			code: Some(code.to_vec()),
+			top,
+			children,
 		}
 	}
 
 	/// Insert key/value
 	pub fn insert(&mut self, k: Vec<u8>, v: Vec<u8>) -> Option<Vec<u8>> {
-		self.inner.insert(k, v)
+		self.top.insert(k, v)
 	}
-}
 
-impl ::std::fmt::Debug for BasicExternalities {
-	fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
-		write!(f, "{:?}", self.inner)
+	/// Consume self and returns inner storages
+	pub fn into_storages(self) -> (
+		HashMap<Vec<u8>, Vec<u8>>,
+		HashMap<Vec<u8>, HashMap<Vec<u8>, Vec<u8>>>,
+	) {
+		(self.top, self.children)
 	}
 }
 
 impl PartialEq for BasicExternalities {
 	fn eq(&self, other: &BasicExternalities) -> bool {
-		self.inner.eq(&other.inner)
+		self.top.eq(&other.top) && self.children.eq(&other.children)
 	}
 }
 
 impl FromIterator<(Vec<u8>, Vec<u8>)> for BasicExternalities {
 	fn from_iter<I: IntoIterator<Item=(Vec<u8>, Vec<u8>)>>(iter: I) -> Self {
-		let mut t = Self::new(Default::default());
-		t.inner.extend(iter);
+		let mut t = Self::default();
+		t.top.extend(iter);
 		t
 	}
 }
@@ -87,69 +84,84 @@ impl Default for BasicExternalities {
 	fn default() -> Self { Self::new(Default::default()) }
 }
 
-impl From<BasicExternalities> for HashMap<Vec<u8>, Vec<u8>> {
-	fn from(tex: BasicExternalities) -> Self {
-		tex.inner.into()
-	}
-}
-
-impl From< HashMap<Vec<u8>, Vec<u8>> > for BasicExternalities {
+impl From<HashMap<Vec<u8>, Vec<u8>>> for BasicExternalities {
 	fn from(hashmap: HashMap<Vec<u8>, Vec<u8>>) -> Self {
 		BasicExternalities {
-			inner: hashmap,
-			changes: Default::default(),
-			code: None,
+			top: hashmap,
+			children: Default::default(),
 		}
 	}
 }
 
 impl<H: Hasher> Externalities<H> for BasicExternalities where H::Out: Ord {
 	fn storage(&self, key: &[u8]) -> Option<Vec<u8>> {
-		match key {
-			CODE => self.code.clone(),
-			_ => self.inner.get(key).cloned(),
-		}
+		self.top.get(key).cloned()
 	}
 
 	fn original_storage(&self, key: &[u8]) -> Option<Vec<u8>> {
 		Externalities::<H>::storage(self, key)
 	}
 
-	fn child_storage(&self, _storage_key: ChildStorageKey<H>, _key: &[u8]) -> Option<Vec<u8>> {
-		None
+	fn child_storage(&self, storage_key: ChildStorageKey<H>, key: &[u8]) -> Option<Vec<u8>> {
+		self.children.get(storage_key.as_ref()).and_then(|child| child.get(key)).cloned()
 	}
 
 	fn place_storage(&mut self, key: Vec<u8>, maybe_value: Option<Vec<u8>>) {
-		self.changes.set_storage(key.clone(), maybe_value.clone());
-		match key.as_ref() {
-			CODE => self.code = maybe_value,
-			_ => {
-				match maybe_value {
-					Some(value) => { self.inner.insert(key, value); }
-					None => { self.inner.remove(&key); }
-				}
-			}
+		if is_child_storage_key(&key) {
+			warn!(target: "trie", "Refuse to set child storage key via main storage");
+			return;
+		}
+
+		match maybe_value {
+			Some(value) => { self.top.insert(key, value); }
+			None => { self.top.remove(&key); }
 		}
 	}
 
-	fn place_child_storage(&mut self, _storage_key: ChildStorageKey<H>, _key: Vec<u8>, _value: Option<Vec<u8>>) {
+	fn place_child_storage(
+		&mut self,
+		storage_key: ChildStorageKey<H>,
+		key: Vec<u8>,
+		value: Option<Vec<u8>>
+	) {
+		let child_map = self.children.entry(storage_key.into_owned()).or_default();
+		if let Some(value) = value {
+			child_map.insert(key, value);
+		} else {
+			child_map.remove(&key);
+		}
 	}
 
-	fn kill_child_storage(&mut self, _storage_key: ChildStorageKey<H>) { }
+	fn kill_child_storage(&mut self, storage_key: ChildStorageKey<H>) {
+		self.children.remove(storage_key.as_ref());
+	}
 
 	fn clear_prefix(&mut self, prefix: &[u8]) {
-		self.changes.clear_prefix(prefix);
-		self.inner.retain(|key, _| !key.starts_with(prefix));
+		if is_child_storage_key(prefix) {
+			warn!(
+				target: "trie",
+				"Refuse to clear prefix that is part of child storage key via main storage"
+			);
+			return;
+		}
+
+		self.top.retain(|key, _| !key.starts_with(prefix));
 	}
 
 	fn chain_id(&self) -> u64 { 42 }
 
 	fn storage_root(&mut self) -> H::Out {
-		trie_root::<H, _, _, _>(self.inner.clone())
+		trie_root::<H, _, _, _>(self.top.clone())
 	}
 
-	fn child_storage_root(&mut self, _storage_key: ChildStorageKey<H>) -> Vec<u8> {
-		vec![42]
+	fn child_storage_root(&mut self, storage_key: ChildStorageKey<H>) -> Vec<u8> {
+		if let Some(child) = self.children.get(storage_key.as_ref()) {
+			let delta = child.clone().into_iter().map(|(k, v)| (k, Some(v)));
+
+			InMemory::<H>::default().child_storage_root(storage_key.as_ref(), delta).0
+		} else {
+			vec![]
+		}
 	}
 
 	fn storage_changes_root(&mut self, _parent: H::Out) -> Result<Option<H::Out>, ()> {
@@ -165,7 +177,8 @@ impl<H: Hasher> Externalities<H> for BasicExternalities where H::Out: Ord {
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use primitives::{Blake2Hasher, H256};
+	use primitives::{Blake2Hasher, H256, map};
+	use primitives::storage::well_known_keys::CODE;
 	use hex_literal::hex;
 
 	#[test]
@@ -189,4 +202,33 @@ mod tests {
 
 		assert_eq!(&ext.storage(CODE).unwrap(), &code);
 	}
+
+	#[test]
+	fn children_works() {
+		let child_storage = b":child_storage:default:test".to_vec();
+
+		let mut ext = BasicExternalities::new_with_children(
+			Default::default(),
+			map![
+				child_storage.clone() => map![
+					b"doe".to_vec() => b"reindeer".to_vec()
+				]
+			]
+		);
+
+		let ext = &mut ext as &mut dyn Externalities<Blake2Hasher>;
+
+		let child = || ChildStorageKey::from_vec(child_storage.clone()).unwrap();
+
+		assert_eq!(ext.child_storage(child(), b"doe"), Some(b"reindeer".to_vec()));
+
+		ext.set_child_storage(child(), b"dog".to_vec(), b"puppy".to_vec());
+		assert_eq!(ext.child_storage(child(), b"dog"), Some(b"puppy".to_vec()));
+
+		ext.clear_child_storage(child(), b"dog");
+		assert_eq!(ext.child_storage(child(), b"dog"), None);
+
+		ext.kill_child_storage(child());
+		assert_eq!(ext.child_storage(child(), b"doe"), None);
+	}
 }
diff --git a/substrate/core/state-machine/src/testing.rs b/substrate/core/state-machine/src/testing.rs
index 759ffb6b96..4a0e221809 100644
--- a/substrate/core/state-machine/src/testing.rs
+++ b/substrate/core/state-machine/src/testing.rs
@@ -16,7 +16,7 @@
 
 //! Test implementation for Externalities.
 
-use std::collections::{HashMap, BTreeMap};
+use std::collections::{HashMap};
 use std::iter::FromIterator;
 use hash_db::Hasher;
 use crate::backend::{InMemory, Backend};
@@ -32,6 +32,8 @@ use super::{ChildStorageKey, Externalities, OverlayedChanges};
 
 const EXT_NOT_ALLOWED_TO_FAIL: &str = "Externalities not allowed to fail within runtime";
 
+type StorageTuple = (HashMap<Vec<u8>, Vec<u8>>, HashMap<Vec<u8>, HashMap<Vec<u8>, Vec<u8>>>);
+
 /// Simple HashMap-based Externalities impl.
 pub struct TestExternalities<H: Hasher, N: ChangesTrieBlockNumber> {
 	overlay: OverlayedChanges,
@@ -41,28 +43,46 @@ pub struct TestExternalities<H: Hasher, N: ChangesTrieBlockNumber> {
 }
 
 impl<H: Hasher, N: ChangesTrieBlockNumber> TestExternalities<H, N> {
-	/// Create a new instance of `TestExternalities`.
-	pub fn new(inner: HashMap<Vec<u8>, Vec<u8>>) -> Self {
-		Self::new_with_code(&[], inner)
+	/// Create a new instance of `TestExternalities` with storage.
+	pub fn new(storage: HashMap<Vec<u8>, Vec<u8>>) -> Self {
+		Self::new_with_children((storage, Default::default()))
 	}
 
-	/// Create a new instance of `TestExternalities`
-	pub fn new_with_code(code: &[u8], mut inner: HashMap<Vec<u8>, Vec<u8>>) -> Self {
+	/// Create a new instance of `TestExternalities` with storage and children.
+	pub fn new_with_children(storage: StorageTuple) -> Self {
+		Self::new_with_code_with_children(&[], storage)
+	}
+
+	/// Create a new instance of `TestExternalities` with code and storage.
+	pub fn new_with_code(code: &[u8], storage: HashMap<Vec<u8>, Vec<u8>>) -> Self {
+		Self::new_with_code_with_children(code, (storage, Default::default()))
+	}
+
+	/// Create a new instance of `TestExternalities` with code, storage and children.
+	pub fn new_with_code_with_children(code: &[u8], mut storage: StorageTuple) -> Self {
 		let mut overlay = OverlayedChanges::default();
 
+		assert!(storage.0.keys().all(|key| !is_child_storage_key(key)));
+		assert!(storage.1.keys().all(|key| is_child_storage_key(key)));
+
 		super::set_changes_trie_config(
 			&mut overlay,
-			inner.get(&CHANGES_TRIE_CONFIG.to_vec()).cloned(),
+			storage.0.get(&CHANGES_TRIE_CONFIG.to_vec()).cloned(),
 			false,
 		).expect("changes trie configuration is correct in test env; qed");
 
-		inner.insert(HEAP_PAGES.to_vec(), 8u64.encode());
-		inner.insert(CODE.to_vec(), code.to_vec());
+		storage.0.insert(HEAP_PAGES.to_vec(), 8u64.encode());
+		storage.0.insert(CODE.to_vec(), code.to_vec());
+
+		let backend: HashMap<_, _> = storage.1.into_iter()
+			.map(|(keyspace, map)| (Some(keyspace), map))
+			.chain(Some((None, storage.0)).into_iter())
+			.collect();
 
 		TestExternalities {
 			overlay,
 			changes_trie_storage: ChangesTrieInMemoryStorage::new(),
-			backend: inner.into(),
+			backend: backend.into(),
 			offchain: None,
 		}
 	}
@@ -72,17 +92,6 @@ impl<H: Hasher, N: ChangesTrieBlockNumber> TestExternalities<H, N> {
 		self.backend = self.backend.update(vec![(None, k, Some(v))]);
 	}
 
-	/// Iter to all pairs in key order
-	pub fn iter_pairs_in_order(&self) -> impl Iterator<Item=(Vec<u8>, Vec<u8>)> {
-		self.backend.pairs().iter()
-			.map(|&(ref k, ref v)| (k.to_vec(), Some(v.to_vec())))
-			.chain(self.overlay.committed.top.clone().into_iter().map(|(k, v)| (k, v.value)))
-			.chain(self.overlay.prospective.top.clone().into_iter().map(|(k, v)| (k, v.value)))
-			.collect::<BTreeMap<_, _>>()
-			.into_iter()
-			.filter_map(|(k, maybe_val)| maybe_val.map(|val| (k, val)))
-	}
-
 	/// Set offchain externaltiies.
 	pub fn set_offchain_externalities(&mut self, offchain: impl offchain::Externalities + 'static) {
 		self.offchain = Some(Box::new(offchain));
@@ -92,9 +101,26 @@ impl<H: Hasher, N: ChangesTrieBlockNumber> TestExternalities<H, N> {
 	pub fn changes_trie_storage(&mut self) -> &mut ChangesTrieInMemoryStorage<H, N> {
 		&mut self.changes_trie_storage
 	}
+
+	/// Return a new backend with all pending value.
+	pub fn commit_all(&self) -> InMemory<H> {
+		let top = self.overlay.committed.top.clone().into_iter()
+			.chain(self.overlay.prospective.top.clone().into_iter())
+			.map(|(k, v)| (None, k, v.value));
+
+		let children = self.overlay.committed.children.clone().into_iter()
+			.chain(self.overlay.prospective.children.clone().into_iter())
+			.flat_map(|(keyspace, map)| {
+				map.1.into_iter()
+					.map(|(k, v)| (Some(keyspace.clone()), k, v))
+					.collect::<Vec<_>>()
+			});
+
+		self.backend.update(top.chain(children).collect())
+	}
 }
 
-impl<H: Hasher, N: ChangesTrieBlockNumber> ::std::fmt::Debug for TestExternalities<H, N> {
+impl<H: Hasher, N: ChangesTrieBlockNumber> std::fmt::Debug for TestExternalities<H, N> {
 	fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
 		write!(f, "overlay: {:?}\nbackend: {:?}", self.overlay, self.backend.pairs())
 	}
@@ -104,15 +130,13 @@ impl<H: Hasher, N: ChangesTrieBlockNumber> PartialEq for TestExternalities<H, N>
 	/// This doesn't test if they are in the same state, only if they contains the
 	/// same data at this state
 	fn eq(&self, other: &TestExternalities<H, N>) -> bool {
-		self.iter_pairs_in_order().eq(other.iter_pairs_in_order())
+		self.commit_all().eq(&other.commit_all())
 	}
 }
 
 impl<H: Hasher, N: ChangesTrieBlockNumber> FromIterator<(Vec<u8>, Vec<u8>)> for TestExternalities<H, N> {
 	fn from_iter<I: IntoIterator<Item=(Vec<u8>, Vec<u8>)>>(iter: I) -> Self {
-		let mut t = Self::new(Default::default());
-		t.backend = t.backend.update(iter.into_iter().map(|(k, v)| (None, k, Some(v))).collect());
-		t
+		Self::new(iter.into_iter().collect())
 	}
 }
 
@@ -120,15 +144,15 @@ impl<H: Hasher, N: ChangesTrieBlockNumber> Default for TestExternalities<H, N> {
 	fn default() -> Self { Self::new(Default::default()) }
 }
 
-impl<H: Hasher, N: ChangesTrieBlockNumber> From<TestExternalities<H, N>> for HashMap<Vec<u8>, Vec<u8>> {
-	fn from(tex: TestExternalities<H, N>) -> Self {
-		tex.iter_pairs_in_order().collect()
+impl<H: Hasher, N: ChangesTrieBlockNumber> From<HashMap<Vec<u8>, Vec<u8>>> for TestExternalities<H, N> {
+	fn from(hashmap: HashMap<Vec<u8>, Vec<u8>>) -> Self {
+		Self::from_iter(hashmap)
 	}
 }
 
-impl<H: Hasher, N: ChangesTrieBlockNumber> From< HashMap<Vec<u8>, Vec<u8>> > for TestExternalities<H, N> {
-	fn from(hashmap: HashMap<Vec<u8>, Vec<u8>>) -> Self {
-		Self::from_iter(hashmap)
+impl<H: Hasher, N: ChangesTrieBlockNumber> From<StorageTuple> for TestExternalities<H, N> {
+	fn from(storage: StorageTuple) -> Self {
+		Self::new_with_children(storage)
 	}
 }
 
diff --git a/substrate/node/executor/src/lib.rs b/substrate/node/executor/src/lib.rs
index 9cf85c2357..1d6f1d1794 100644
--- a/substrate/node/executor/src/lib.rs
+++ b/substrate/node/executor/src/lib.rs
@@ -297,7 +297,7 @@ mod tests {
 	}
 
 	fn new_test_ext(code: &[u8], support_changes_trie: bool) -> TestExternalities<Blake2Hasher> {
-		let mut ext = TestExternalities::new_with_code(code, GenesisConfig {
+		let mut ext = TestExternalities::new_with_code_with_children(code, GenesisConfig {
 			aura: Some(Default::default()),
 			system: Some(SystemConfig {
 				changes_trie_config: if support_changes_trie { Some(ChangesTrieConfiguration {
@@ -353,7 +353,7 @@ mod tests {
 			grandpa: Some(GrandpaConfig {
 				authorities: vec![],
 			}),
-		}.build_storage().unwrap().0);
+		}.build_storage().unwrap());
 		ext.changes_trie_storage().insert(0, GENESIS_HASH.into(), Default::default());
 		ext
 	}
diff --git a/substrate/srml/contracts/src/tests.rs b/substrate/srml/contracts/src/tests.rs
index 0d4688f9a7..b195a74bf4 100644
--- a/substrate/srml/contracts/src/tests.rs
+++ b/substrate/srml/contracts/src/tests.rs
@@ -260,26 +260,16 @@ impl ExtBuilder {
 	}
 	pub fn build(self) -> runtime_io::TestExternalities<Blake2Hasher> {
 		self.set_associated_consts();
-		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap().0;
-		t.extend(
-			balances::GenesisConfig::<Test> {
-				balances: vec![],
-				vesting: vec![],
-			}
-			.build_storage()
-			.unwrap()
-			.0,
-		);
-		t.extend(
-			GenesisConfig::<Test> {
-				current_schedule: Default::default(),
-				gas_price: self.gas_price,
-			}
-			.build_storage()
-			.unwrap()
-			.0,
-		);
-		runtime_io::TestExternalities::new(t)
+		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
+		balances::GenesisConfig::<Test> {
+			balances: vec![],
+			vesting: vec![],
+		}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
+		GenesisConfig::<Test> {
+			current_schedule: Default::default(),
+			gas_price: self.gas_price,
+		}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
+		runtime_io::TestExternalities::new_with_children(t)
 	}
 }
 
diff --git a/substrate/srml/council/src/lib.rs b/substrate/srml/council/src/lib.rs
index bc66862a2a..8a52f9ec00 100644
--- a/substrate/srml/council/src/lib.rs
+++ b/substrate/srml/council/src/lib.rs
@@ -238,8 +238,8 @@ mod tests {
 		}
 		pub fn build(self) -> runtime_io::TestExternalities<Blake2Hasher> {
 			self.set_associated_consts();
-			let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap().0;
-			t.extend(balances::GenesisConfig::<Test>{
+			let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
+			balances::GenesisConfig::<Test>{
 				balances: vec![
 					(1, 10 * self.balance_factor),
 					(2, 20 * self.balance_factor),
@@ -249,8 +249,8 @@ mod tests {
 					(6, 60 * self.balance_factor)
 				],
 				vesting: vec![],
-			}.build_storage().unwrap().0);
-			t.extend(seats::GenesisConfig::<Test> {
+			}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
+			seats::GenesisConfig::<Test> {
 				active_council: if self.with_council { vec![
 					(1, 10),
 					(2, 10),
@@ -259,8 +259,8 @@ mod tests {
 				desired_seats: 2,
 				presentation_duration: 2,
 				term_duration: 5,
-			}.build_storage().unwrap().0);
-			runtime_io::TestExternalities::new(t)
+			}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
+			runtime_io::TestExternalities::new_with_children(t)
 		}
 	}
 
diff --git a/substrate/srml/democracy/src/lib.rs b/substrate/srml/democracy/src/lib.rs
index d0c71a63d6..16a6d3b5e2 100644
--- a/substrate/srml/democracy/src/lib.rs
+++ b/substrate/srml/democracy/src/lib.rs
@@ -1044,13 +1044,13 @@ mod tests {
 	}
 
 	fn new_test_ext() -> runtime_io::TestExternalities<Blake2Hasher> {
-		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap().0;
-		t.extend(balances::GenesisConfig::<Test>{
+		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
+		balances::GenesisConfig::<Test>{
 			balances: vec![(1, 10), (2, 20), (3, 30), (4, 40), (5, 50), (6, 60)],
 			vesting: vec![],
-		}.build_storage().unwrap().0);
-		t.extend(GenesisConfig::default().build_storage().unwrap().0);
-		runtime_io::TestExternalities::new(t)
+		}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
+		GenesisConfig::default().assimilate_storage(&mut t.0, &mut t.1).unwrap();
+		runtime_io::TestExternalities::new_with_children(t)
 	}
 
 	type System = system::Module<Test>;
diff --git a/substrate/srml/executive/src/lib.rs b/substrate/srml/executive/src/lib.rs
index 2b5ebdd88c..6ee9da3c54 100644
--- a/substrate/srml/executive/src/lib.rs
+++ b/substrate/srml/executive/src/lib.rs
@@ -482,13 +482,13 @@ mod tests {
 
 	#[test]
 	fn balance_transfer_dispatch_works() {
-		let mut t = system::GenesisConfig::default().build_storage::<Runtime>().unwrap().0;
-		t.extend(balances::GenesisConfig::<Runtime> {
+		let mut t = system::GenesisConfig::default().build_storage::<Runtime>().unwrap();
+		balances::GenesisConfig::<Runtime> {
 			balances: vec![(1, 111)],
 			vesting: vec![],
-		}.build_storage().unwrap().0);
+		}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
 		let xt = primitives::testing::TestXt(Some(1), 0, Call::transfer(2, 69));
-		let mut t = runtime_io::TestExternalities::<Blake2Hasher>::new(t);
+		let mut t = runtime_io::TestExternalities::<Blake2Hasher>::new_with_children(t);
 		with_externalities(&mut t, || {
 			Executive::initialize_block(&Header::new(
 				1,
@@ -504,8 +504,8 @@ mod tests {
 	}
 
 	fn new_test_ext() -> runtime_io::TestExternalities<Blake2Hasher> {
-		let mut t = system::GenesisConfig::default().build_storage::<Runtime>().unwrap().0;
-		t.extend(balances::GenesisConfig::<Runtime>::default().build_storage().unwrap().0);
+		let mut t = system::GenesisConfig::default().build_storage::<Runtime>().unwrap();
+		balances::GenesisConfig::<Runtime>::default().assimilate_storage(&mut t.0, &mut t.1).unwrap();
 		t.into()
 	}
 
diff --git a/substrate/srml/finality-tracker/src/lib.rs b/substrate/srml/finality-tracker/src/lib.rs
index 98a8a175af..f9ccc36346 100644
--- a/substrate/srml/finality-tracker/src/lib.rs
+++ b/substrate/srml/finality-tracker/src/lib.rs
@@ -327,8 +327,8 @@ mod tests {
 
 	#[test]
 	fn median_works() {
-		let t = system::GenesisConfig::default().build_storage::<Test>().unwrap().0;
-		with_externalities(&mut TestExternalities::new(t), || {
+		let t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
+		with_externalities(&mut TestExternalities::new_with_children(t), || {
 			FinalityTracker::update_hint(Some(500));
 			assert_eq!(FinalityTracker::median(), 250);
 			assert!(NOTIFICATIONS.with(|n| n.borrow().is_empty()));
@@ -337,8 +337,8 @@ mod tests {
 
 	#[test]
 	fn notifies_when_stalled() {
-		let t = system::GenesisConfig::default().build_storage::<Test>().unwrap().0;
-		with_externalities(&mut TestExternalities::new(t), || {
+		let t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
+		with_externalities(&mut TestExternalities::new_with_children(t), || {
 			let mut parent_hash = System::parent_hash();
 			for i in 2..106 {
 				System::initialize(&i, &parent_hash, &Default::default(), &Default::default());
@@ -356,8 +356,8 @@ mod tests {
 
 	#[test]
 	fn recent_notifications_prevent_stalling() {
-		let t = system::GenesisConfig::default().build_storage::<Test>().unwrap().0;
-		with_externalities(&mut TestExternalities::new(t), || {
+		let t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
+		with_externalities(&mut TestExternalities::new_with_children(t), || {
 			let mut parent_hash = System::parent_hash();
 			for i in 2..106 {
 				System::initialize(&i, &parent_hash, &Default::default(), &Default::default());
diff --git a/substrate/srml/session/src/lib.rs b/substrate/srml/session/src/lib.rs
index 744e00fa10..fe7a959724 100644
--- a/substrate/srml/session/src/lib.rs
+++ b/substrate/srml/session/src/lib.rs
@@ -563,17 +563,16 @@ mod tests {
 	};
 
 	fn new_test_ext() -> runtime_io::TestExternalities<Blake2Hasher> {
-		TEST_SESSION_CHANGED.with(|l| *l.borrow_mut() = false);
-		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap().0;
-		t.extend(timestamp::GenesisConfig::<Test> {
+		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
+		timestamp::GenesisConfig::<Test> {
 			minimum_period: 5,
-		}.build_storage().unwrap().0);
-		t.extend(GenesisConfig::<Test> {
+		}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
+		GenesisConfig::<Test> {
 			keys: NEXT_VALIDATORS.with(|l|
 				l.borrow().iter().cloned().map(|i| (i, UintAuthorityId(i))).collect()
 			),
-		}.build_storage().unwrap().0);
-		runtime_io::TestExternalities::new(t)
+		}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
+		runtime_io::TestExternalities::new_with_children(t)
 	}
 
 	fn initialize_block(block: u64) {
diff --git a/substrate/srml/timestamp/src/lib.rs b/substrate/srml/timestamp/src/lib.rs
index c915f6b95d..7bffac0db9 100644
--- a/substrate/srml/timestamp/src/lib.rs
+++ b/substrate/srml/timestamp/src/lib.rs
@@ -364,12 +364,12 @@ mod tests {
 
 	#[test]
 	fn timestamp_works() {
-		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap().0;
-		t.extend(GenesisConfig::<Test> {
+		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
+		GenesisConfig::<Test> {
 			minimum_period: 5,
-		}.build_storage().unwrap().0);
+		}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
 
-		with_externalities(&mut TestExternalities::new(t), || {
+		with_externalities(&mut TestExternalities::new_with_children(t), || {
 			Timestamp::set_timestamp(42);
 			assert_ok!(Timestamp::dispatch(Call::set(69), Origin::NONE));
 			assert_eq!(Timestamp::now(), 69);
@@ -379,12 +379,12 @@ mod tests {
 	#[test]
 	#[should_panic(expected = "Timestamp must be updated only once in the block")]
 	fn double_timestamp_should_fail() {
-		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap().0;
-		t.extend(GenesisConfig::<Test> {
+		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
+		GenesisConfig::<Test> {
 			minimum_period: 5,
-		}.build_storage().unwrap().0);
+		}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
 
-		with_externalities(&mut TestExternalities::new(t), || {
+		with_externalities(&mut TestExternalities::new_with_children(t), || {
 			Timestamp::set_timestamp(42);
 			assert_ok!(Timestamp::dispatch(Call::set(69), Origin::NONE));
 			let _ = Timestamp::dispatch(Call::set(70), Origin::NONE);
@@ -394,12 +394,12 @@ mod tests {
 	#[test]
 	#[should_panic(expected = "Timestamp must increment by at least <MinimumPeriod> between sequential blocks")]
 	fn block_period_minimum_enforced() {
-		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap().0;
-		t.extend(GenesisConfig::<Test> {
+		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
+		GenesisConfig::<Test> {
 			minimum_period: 5,
-		}.build_storage().unwrap().0);
+		}.assimilate_storage(&mut t.0, &mut t.1).unwrap();
 
-		with_externalities(&mut TestExternalities::new(t), || {
+		with_externalities(&mut TestExternalities::new_with_children(t), || {
 			Timestamp::set_timestamp(42);
 			let _ = Timestamp::dispatch(Call::set(46), Origin::NONE);
 		});