diff --git a/substrate/substrate/client/src/block_builder.rs b/substrate/substrate/client/src/block_builder.rs
index ebd8ec6c47..8c2d0c0628 100644
--- a/substrate/substrate/client/src/block_builder.rs
+++ b/substrate/substrate/client/src/block_builder.rs
@@ -42,24 +42,29 @@ impl<B, E> BlockBuilder<B, E> where
 	E: CodeExecutor + Clone,
 	error::Error: From<<<B as backend::Backend>::State as state_machine::backend::Backend>::Error>,
 {
-	/// Create a new instance of builder from the given client.
+	/// Create a new instance of builder from the given client, building on the latest block.
 	pub fn new(client: &Client<B, E>) -> error::Result<Self> {
 		let best = (client.info().map(|i| i.chain.best_number)?..1)
 			.find(|&n| if let Ok(BlockStatus::InChain) = client.block_status(&BlockId::Number(n))
 				{ true } else { false })
 			.unwrap_or(0);
+		Self::at_block(&BlockId::Number(best), client)
+	}
 
+	/// Create a new instance of builder from the given client using a particular block's ID to
+	/// build upon.
+	pub fn at_block(block_id: &BlockId, client: &Client<B, E>) -> error::Result<Self> {
 		Ok(BlockBuilder {
 			header: Header {
-				number: best + 1,
-				parent_hash: client.block_hash(best)?.expect("We already ascertained this is InChain before; qed"),
+				number: client.block_number_from_id(block_id)?.ok_or(error::ErrorKind::UnknownBlock(*block_id))? + 1,
+				parent_hash: client.block_hash_from_id(block_id)?.ok_or(error::ErrorKind::UnknownBlock(*block_id))?,
 				state_root: Default::default(),
 				transaction_root: Default::default(),
 				digest: Default::default(),
 			},
 			transactions: Default::default(),
 			executor: client.clone_executor(),
-			state: client.state_at(&BlockId::Number(best))?,
+			state: client.state_at(block_id)?,
 			changes: Default::default(),
 		})
 	}
diff --git a/substrate/substrate/client/src/lib.rs b/substrate/substrate/client/src/lib.rs
index 1a7d4ceb62..f347dda3cb 100644
--- a/substrate/substrate/client/src/lib.rs
+++ b/substrate/substrate/client/src/lib.rs
@@ -43,7 +43,6 @@ pub mod block_builder;
 
 pub use blockchain::Info as ChainInfo;
 pub use blockchain::BlockId;
-pub use block_builder::BlockBuilder;
 
 use primitives::{block, AuthorityId};
 use primitives::storage::{StorageKey, StorageData};
@@ -202,8 +201,13 @@ impl<B, E> Client<B, E> where
 	}
 
 	/// Create a new block, built on the head of the chain.
-	pub fn new_block(&self) -> error::Result<BlockBuilder<B, E>> where E: Clone {
-		BlockBuilder::new(self)
+	pub fn new_block(&self) -> error::Result<block_builder::BlockBuilder<B, E>> where E: Clone {
+		block_builder::BlockBuilder::new(self)
+	}
+
+	/// Create a new block, built on top of `parent`.
+	pub fn new_block_at(&self, parent: &BlockId) -> error::Result<block_builder::BlockBuilder<B, E>> where E: Clone {
+		block_builder::BlockBuilder::at_block(parent, &self)
 	}
 
 	/// Queue a block for import.
@@ -249,6 +253,22 @@ impl<B, E> Client<B, E> where
 		self.backend.blockchain().hash(block_number)
 	}
 
+	/// Convert an arbitrary block ID into a block hash.
+	pub fn block_hash_from_id(&self, id: &BlockId) -> error::Result<Option<block::HeaderHash>> {
+		match *id {
+			BlockId::Hash(h) => Ok(Some(h)),
+			BlockId::Number(n) => self.block_hash(n),
+		}
+	}
+
+	/// Convert an arbitrary block ID into a block hash.
+	pub fn block_number_from_id(&self, id: &BlockId) -> error::Result<Option<block::Number>> {
+		match *id {
+			BlockId::Hash(_) => Ok(self.header(id)?.map(|h| h.number)),
+			BlockId::Number(n) => Ok(Some(n)),
+		}
+	}
+
 	/// Get block header by id.
 	pub fn header(&self, id: &BlockId) -> error::Result<Option<block::Header>> {
 		self.backend.blockchain().header(*id)