diff --git a/Cargo.toml b/Cargo.toml
index 9c0490023d..5f183328aa 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -25,6 +25,8 @@ runtime_primitives = { git = "https://github.com/paritytech/substrate/", package
 serde = { version = "1.0", features = ["derive"] }
 sr-version = { git = "https://github.com/paritytech/substrate/", package = "sr-version" }
 srml-system = { git = "https://github.com/paritytech/substrate/", package = "srml-system" }
+srml-balances = { git = "https://github.com/paritytech/substrate/", package = "srml-balances" }
+srml-contracts = { git = "https://github.com/paritytech/substrate/", package = "srml-contracts" }
 substrate-rpc-api = { git = "https://github.com/paritytech/substrate/", package = "substrate-rpc-api" }
 substrate-rpc-primitives = { git = "https://github.com/paritytech/substrate/", package = "substrate-rpc-primitives" }
 substrate-primitives = { git = "https://github.com/paritytech/substrate/", package = "substrate-primitives" }
@@ -34,8 +36,6 @@ url = "1.7"
 [dev-dependencies]
 env_logger = "0.6"
 node-runtime = { git = "https://github.com/paritytech/substrate/", package = "node-runtime" }
-srml-balances = { git = "https://github.com/paritytech/substrate/", package = "srml-balances" }
-srml-contracts = { git = "https://github.com/paritytech/substrate/", package = "srml-contracts" }
 substrate-keyring = { git = "https://github.com/paritytech/substrate/", package = "substrate-keyring" }
 tokio = "0.1"
 wabt = "0.9.0"
diff --git a/src/error.rs b/src/error.rs
index 9557bcb6a2..9e9cfb98cb 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -20,6 +20,7 @@ use crate::{
 };
 use jsonrpc_core_client::RpcError;
 use parity_scale_codec::Error as CodecError;
+use runtime_primitives::transaction_validity::TransactionValidityError;
 use std::io::Error as IoError;
 use substrate_primitives::crypto::SecretStringError;
 
@@ -39,6 +40,9 @@ pub enum Error {
     SecretString(SecretStringError),
     /// Metadata error.
     Metadata(MetadataError),
+    /// Extrinsic validity error
+    #[display(fmt = "Transaction Validity Error: {:?}", _0)]
+    Invalid(TransactionValidityError),
     /// Other error.
     Other(String),
 }
diff --git a/src/extrinsic.rs b/src/extrinsic.rs
new file mode 100644
index 0000000000..1f51cfdc2f
--- /dev/null
+++ b/src/extrinsic.rs
@@ -0,0 +1,263 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+// This file is part of substrate-subxt.
+//
+// subxt is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// subxt is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with substrate-subxt.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::srml::{
+    balances::Balances,
+    system::System,
+};
+use parity_scale_codec::{
+    Codec,
+    Decode,
+    Encode,
+};
+use runtime_primitives::{
+    generic::{
+        Era,
+        SignedPayload,
+        UncheckedExtrinsic,
+    },
+    traits::SignedExtension,
+    transaction_validity::TransactionValidityError,
+};
+use std::marker::PhantomData;
+use substrate_primitives::Pair;
+
+/// SignedExtra checks copied from substrate, in order to remove requirement to implement
+/// substrate's `srml_system::Trait`
+
+/// Ensure the runtime version registered in the transaction is the same as at present.
+///
+/// # Note
+///
+/// This is modified from the substrate version to allow passing in of the version, which is
+/// returned via `additional_signed()`.
+#[derive(Encode, Decode, Clone, Eq, PartialEq, Debug)]
+pub struct CheckVersion<T: System + Send + Sync>(
+    PhantomData<T>,
+    /// Local version to be used for `AdditionalSigned`
+    #[codec(skip)]
+    u32,
+);
+
+impl<T> SignedExtension for CheckVersion<T> where T: System + Send + Sync {
+    type AccountId = u64;
+    type Call = ();
+    type AdditionalSigned = u32;
+    type Pre = ();
+    fn additional_signed(&self) -> Result<Self::AdditionalSigned, TransactionValidityError> {
+        Ok(self.1)
+    }
+}
+
+/// Check genesis hash
+///
+/// # Note
+///
+/// This is modified from the substrate version to allow passing in of the genesis hash, which is
+/// returned via `additional_signed()`.
+#[derive(Encode, Decode, Clone, Eq, PartialEq, Debug)]
+pub struct CheckGenesis<T: System + Send + Sync>(
+    PhantomData<T>,
+    /// Local genesis hash to be used for `AdditionalSigned`
+    #[codec(skip)]
+    T::Hash,
+);
+
+impl<T> SignedExtension for CheckGenesis<T> where T: System + Send + Sync {
+    type AccountId = u64;
+    type Call = ();
+    type AdditionalSigned = T::Hash;
+    type Pre = ();
+    fn additional_signed(&self) -> Result<Self::AdditionalSigned, TransactionValidityError> {
+        Ok(self.1)
+    }
+}
+
+/// Check for transaction mortality.
+///
+/// # Note
+///
+/// This is modified from the substrate version to allow passing in of the genesis hash, which is
+/// returned via `additional_signed()`. It assumes therefore `Era::Immortal` (The transaction is
+/// valid forever)
+#[derive(Encode, Decode, Clone, Eq, PartialEq, Debug)]
+pub struct CheckEra<T: System + Send + Sync>(
+    /// The default structure for the Extra encoding
+    (Era, PhantomData<T>),
+    /// Local genesis hash to be used for `AdditionalSigned`
+    #[codec(skip)]
+    T::Hash,
+);
+
+impl<T> SignedExtension for CheckEra<T> where T: System + Send + Sync {
+    type AccountId = u64;
+    type Call = ();
+    type AdditionalSigned = T::Hash;
+    type Pre = ();
+    fn additional_signed(&self) -> Result<Self::AdditionalSigned, TransactionValidityError> {
+        Ok(self.1)
+    }
+}
+
+/// Nonce check and increment to give replay protection for transactions.
+#[derive(Encode, Decode, Clone, Eq, PartialEq, Debug)]
+pub struct CheckNonce<T: System + Send + Sync>(#[codec(compact)] T::Index);
+
+impl<T> SignedExtension for CheckNonce<T> where T: System + Send + Sync {
+    type AccountId = u64;
+    type Call = ();
+    type AdditionalSigned = ();
+    type Pre = ();
+    fn additional_signed(&self) -> Result<Self::AdditionalSigned, TransactionValidityError> {
+        Ok(())
+    }
+}
+
+/// Resource limit check.
+#[derive(Encode, Decode, Clone, Eq, PartialEq, Debug)]
+pub struct CheckWeight<T: System + Send + Sync>(PhantomData<T>);
+
+impl<T> SignedExtension for CheckWeight<T> where T: System + Send + Sync {
+    type AccountId = u64;
+    type Call = ();
+    type AdditionalSigned = ();
+    type Pre = ();
+    fn additional_signed(&self) -> Result<Self::AdditionalSigned, TransactionValidityError> {
+        Ok(())
+    }
+}
+
+/// Require the transactor pay for themselves and maybe include a tip to gain additional priority
+/// in the queue.
+#[derive(Encode, Decode, Clone, Eq, PartialEq, Debug)]
+pub struct TakeFees<T: Balances>(#[codec(compact)] T::Balance);
+
+impl<T> SignedExtension for TakeFees<T> where T: Balances + Send + Sync {
+    type AccountId = u64;
+    type Call = ();
+    type AdditionalSigned = ();
+    type Pre = ();
+    fn additional_signed(&self) -> Result<Self::AdditionalSigned, TransactionValidityError> {
+        Ok(())
+    }
+}
+
+/// Checks if a transaction would exhausts the block gas limit.
+#[derive(Encode, Decode, Clone, Eq, PartialEq, Debug)]
+pub struct CheckBlockGasLimit<T: System + Send + Sync>(PhantomData<T>);
+
+impl<T> SignedExtension for CheckBlockGasLimit<T> where T: System + Send + Sync {
+    type AccountId = u64;
+    type Call = ();
+    type AdditionalSigned = ();
+    type Pre = ();
+    fn additional_signed(&self) -> Result<Self::AdditionalSigned, TransactionValidityError> {
+        Ok(())
+    }
+}
+
+pub trait SignedExtra<T> {
+    type Extra: SignedExtension;
+
+    fn extra(&self) -> Self::Extra;
+}
+
+#[derive(Encode, Decode, Clone, Eq, PartialEq, Debug)]
+pub struct DefaultExtra<T: System> {
+    version: u32,
+    nonce: T::Index,
+    genesis_hash: T::Hash,
+}
+
+impl<T: System + Balances + Send + Sync> DefaultExtra<T> {
+    pub fn new(version: u32, nonce: T::Index, genesis_hash: T::Hash) -> Self {
+        DefaultExtra {
+            version,
+            nonce,
+            genesis_hash,
+        }
+    }
+}
+
+impl<T: System + Balances + Send + Sync> SignedExtra<T> for DefaultExtra<T> {
+    type Extra = (
+        CheckVersion<T>,
+        CheckGenesis<T>,
+        CheckEra<T>,
+        CheckNonce<T>,
+        CheckWeight<T>,
+        TakeFees<T>,
+        CheckBlockGasLimit<T>,
+    );
+
+    fn extra(&self) -> Self::Extra {
+        (
+            CheckVersion(PhantomData, self.version),
+            CheckGenesis(PhantomData, self.genesis_hash),
+            CheckEra((Era::Immortal, PhantomData), self.genesis_hash),
+            CheckNonce(self.nonce),
+            CheckWeight(PhantomData),
+            TakeFees(<T as Balances>::Balance::default()),
+            CheckBlockGasLimit(PhantomData),
+        )
+    }
+}
+
+impl<T: System + Balances + Send + Sync> SignedExtension for DefaultExtra<T> {
+    type AccountId = T::AccountId;
+    type Call = ();
+    type AdditionalSigned =
+        <<Self as SignedExtra<T>>::Extra as SignedExtension>::AdditionalSigned;
+    type Pre = ();
+
+    fn additional_signed(
+        &self,
+    ) -> Result<Self::AdditionalSigned, TransactionValidityError> {
+        self.extra().additional_signed()
+    }
+}
+
+pub fn create_and_sign<T: System + Send + Sync, C, P, E>(
+    signer: P,
+    call: C,
+    extra: E,
+) -> Result<
+    UncheckedExtrinsic<
+        T::Address,
+        C,
+        P::Signature,
+        <E as SignedExtra<T>>::Extra,
+    >,
+    TransactionValidityError,
+>
+where
+    P: Pair,
+    P::Public: Into<T::Address>,
+    P::Signature: Codec,
+    C: Encode,
+    E: SignedExtra<T> + SignedExtension,
+{
+    let raw_payload = SignedPayload::new(call, extra.extra())?;
+    let signature = raw_payload.using_encoded(|payload| signer.sign(payload));
+    let (call, extra, _) = raw_payload.deconstruct();
+
+    Ok(UncheckedExtrinsic::new_signed(
+        call,
+        signer.public().into(),
+        signature.into(),
+        extra,
+    ))
+}
diff --git a/src/lib.rs b/src/lib.rs
index 166e8a2fab..4cbc3e66f8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -31,19 +31,14 @@ use metadata::Metadata;
 use parity_scale_codec::{
     Codec,
     Decode,
-    Encode,
-};
-use runtime_primitives::{
-    generic::UncheckedExtrinsic,
-    traits::StaticLookup,
 };
+use runtime_primitives::generic::UncheckedExtrinsic;
 use sr_version::RuntimeVersion;
 use std::{
     convert::TryFrom,
     marker::PhantomData,
 };
 use substrate_primitives::{
-    blake2_256,
     storage::{
         StorageChangeSet,
         StorageKey,
@@ -55,6 +50,10 @@ use url::Url;
 use crate::{
     codec::Encoded,
     events::EventsDecoder,
+    extrinsic::{
+        DefaultExtra,
+        SignedExtra,
+    },
     metadata::MetadataError,
     rpc::{
         BlockNumber,
@@ -76,6 +75,7 @@ use crate::{
 mod codec;
 mod error;
 mod events;
+mod extrinsic;
 mod metadata;
 mod rpc;
 mod srml;
@@ -235,7 +235,7 @@ impl<T: System + Balances + 'static> Client<T> {
     ) -> impl Future<Item = XtBuilder<T, P>, Error = Error>
     where
         P: Pair,
-        P::Public: Into<T::AccountId> + Into<<T::Lookup as StaticLookup>::Source>,
+        P::Public: Into<T::AccountId> + Into<T::Address>,
         P::Signature: Codec,
     {
         let client = self.clone();
@@ -324,10 +324,10 @@ where
     }
 }
 
-impl<T: System + Balances + 'static, P> XtBuilder<T, P, Valid>
+impl<T: System + Balances + Send + Sync + 'static, P> XtBuilder<T, P, Valid>
 where
     P: Pair,
-    P::Public: Into<<T::Lookup as StaticLookup>::Source>,
+    P::Public: Into<T::Address>,
     P::Signature: Codec,
 {
     /// Creates and signs an Extrinsic for the supplied `Call`
@@ -335,16 +335,16 @@ where
         &self,
     ) -> Result<
         UncheckedExtrinsic<
-            <T::Lookup as StaticLookup>::Source,
+            T::Address,
             Encoded,
             P::Signature,
-            T::SignedExtra,
+            <DefaultExtra<T> as SignedExtra<T>>::Extra,
         >,
-        MetadataError,
+        Error,
     >
     where
         P: Pair,
-        P::Public: Into<<T::Lookup as StaticLookup>::Source>,
+        P::Public: Into<T::Address>,
         P::Signature: Codec,
     {
         let signer = self.signer.clone();
@@ -362,27 +362,9 @@ where
             account_nonce
         );
 
-        let extra = T::extra(account_nonce);
-        let raw_payload = (
-            call.clone(),
-            extra.clone(),
-            version,
-            (&genesis_hash, &genesis_hash),
-        );
-        let signature = raw_payload.using_encoded(|payload| {
-            if payload.len() > 256 {
-                signer.sign(&blake2_256(payload)[..])
-            } else {
-                signer.sign(payload)
-            }
-        });
-
-        Ok(UncheckedExtrinsic::new_signed(
-            raw_payload.0,
-            signer.public().into(),
-            signature.into(),
-            extra,
-        ))
+        let extra = extrinsic::DefaultExtra::new(version, account_nonce, genesis_hash);
+        let xt = extrinsic::create_and_sign(signer, call, extra)?;
+        Ok(xt)
     }
 
     /// Submits a transaction to the chain.
@@ -427,14 +409,11 @@ mod tests {
             BalancesStore,
             BalancesXt,
         },
-        contracts::{
-            Contracts,
-            ContractsXt,
-        },
+        contracts::ContractsXt,
     };
     use futures::stream::Stream;
+    use node_runtime::Runtime;
     use parity_scale_codec::Encode;
-    use runtime_primitives::generic::Era;
     use runtime_support::StorageMap;
     use substrate_keyring::AccountKeyring;
     use substrate_primitives::{
@@ -442,47 +421,9 @@ mod tests {
         Pair,
     };
 
-    struct Runtime;
-
-    impl System for Runtime {
-        type Index = <node_runtime::Runtime as srml_system::Trait>::Index;
-        type BlockNumber = <node_runtime::Runtime as srml_system::Trait>::BlockNumber;
-        type Hash = <node_runtime::Runtime as srml_system::Trait>::Hash;
-        type Hashing = <node_runtime::Runtime as srml_system::Trait>::Hashing;
-        type AccountId = <node_runtime::Runtime as srml_system::Trait>::AccountId;
-        type Lookup = <node_runtime::Runtime as srml_system::Trait>::Lookup;
-        type Header = <node_runtime::Runtime as srml_system::Trait>::Header;
-        type Event = <node_runtime::Runtime as srml_system::Trait>::Event;
-
-        type SignedExtra = (
-            srml_system::CheckVersion<node_runtime::Runtime>,
-            srml_system::CheckGenesis<node_runtime::Runtime>,
-            srml_system::CheckEra<node_runtime::Runtime>,
-            srml_system::CheckNonce<node_runtime::Runtime>,
-            srml_system::CheckWeight<node_runtime::Runtime>,
-            srml_balances::TakeFees<node_runtime::Runtime>,
-        );
-        fn extra(nonce: Self::Index) -> Self::SignedExtra {
-            (
-                srml_system::CheckVersion::<node_runtime::Runtime>::new(),
-                srml_system::CheckGenesis::<node_runtime::Runtime>::new(),
-                srml_system::CheckEra::<node_runtime::Runtime>::from(Era::Immortal),
-                srml_system::CheckNonce::<node_runtime::Runtime>::from(nonce),
-                srml_system::CheckWeight::<node_runtime::Runtime>::new(),
-                srml_balances::TakeFees::<node_runtime::Runtime>::from(0),
-            )
-        }
-    }
-
-    impl Balances for Runtime {
-        type Balance = <node_runtime::Runtime as srml_balances::Trait>::Balance;
-    }
-
-    impl Contracts for Runtime {}
-
     type Index = <Runtime as System>::Index;
     type AccountId = <Runtime as System>::AccountId;
-    type Address = <<Runtime as System>::Lookup as StaticLookup>::Source;
+    type Address = <Runtime as System>::Address;
     type Balance = <Runtime as Balances>::Balance;
 
     fn test_setup() -> (tokio::runtime::Runtime, Client<Runtime>) {
diff --git a/src/srml/balances.rs b/src/srml/balances.rs
index 2061f63825..425815f391 100644
--- a/src/srml/balances.rs
+++ b/src/srml/balances.rs
@@ -23,7 +23,6 @@ use runtime_primitives::traits::{
     MaybeSerializeDebug,
     Member,
     SimpleArithmetic,
-    StaticLookup,
 };
 use runtime_support::Parameter;
 use substrate_primitives::Pair;
@@ -41,6 +40,14 @@ pub trait Balances: System {
         + From<<Self as System>::BlockNumber>;
 }
 
+/// Blanket impl for using existing runtime types
+impl<T: srml_system::Trait + srml_balances::Trait + std::fmt::Debug> Balances for T
+where
+    <T as srml_system::Trait>::Header: serde::de::DeserializeOwned,
+{
+    type Balance = T::Balance;
+}
+
 /// The Balances extension trait for the Client.
 pub trait BalancesStore {
     /// Balances type.
@@ -134,7 +141,7 @@ where
     /// of the transfer, the account will be reaped.
     pub fn transfer(
         self,
-        to: <<T as System>::Lookup as StaticLookup>::Source,
+        to: <T as System>::Address,
         amount: <T as Balances>::Balance,
     ) -> Result<Encoded, MetadataError> {
         self.module.call("transfer", (to, compact(amount)))
diff --git a/src/srml/contracts.rs b/src/srml/contracts.rs
index 12873156d2..d6e8e41e0e 100644
--- a/src/srml/contracts.rs
+++ b/src/srml/contracts.rs
@@ -13,7 +13,6 @@ use crate::{
     Valid,
     XtBuilder,
 };
-use runtime_primitives::traits::StaticLookup;
 use substrate_primitives::Pair;
 
 /// Gas units are chosen to be represented by u64 so that gas metering
@@ -23,6 +22,18 @@ pub type Gas = u64;
 /// The subset of the `srml_contracts::Trait` that a client must implement.
 pub trait Contracts: System + Balances {}
 
+/// Blanket impl for using existing runtime types
+impl<
+        T: srml_contracts::Trait
+            + srml_system::Trait
+            + srml_balances::Trait
+            + std::fmt::Debug,
+    > Contracts for T
+where
+    <T as srml_system::Trait>::Header: serde::de::DeserializeOwned,
+{
+}
+
 /// The Contracts extension trait for the XtBuilder.
 pub trait ContractsXt {
     /// Contracts type.
@@ -106,7 +117,7 @@ where
     ///  will be transferred.
     pub fn call(
         &self,
-        dest: <<T as System>::Lookup as StaticLookup>::Source,
+        dest: <T as System>::Address,
         value: <T as Balances>::Balance,
         gas_limit: Gas,
         data: Vec<u8>,
diff --git a/src/srml/system.rs b/src/srml/system.rs
index f377682486..921d3c5b0c 100644
--- a/src/srml/system.rs
+++ b/src/srml/system.rs
@@ -15,27 +15,27 @@ use futures::future::{
     self,
     Future,
 };
+use parity_scale_codec::Codec;
 use runtime_primitives::traits::{
     Bounded,
     CheckEqual,
     Hash,
     Header,
+    MaybeDebug,
     MaybeDisplay,
     MaybeSerializeDebug,
     MaybeSerializeDebugButNotDeserialize,
     Member,
-    SignedExtension,
     SimpleArithmetic,
     SimpleBitOps,
     StaticLookup,
 };
 use runtime_support::Parameter;
 use serde::de::DeserializeOwned;
-use srml_system::Event;
 use substrate_primitives::Pair;
 
 /// The subset of the `srml_system::Trait` that a client must implement.
-pub trait System {
+pub trait System: 'static + Eq + Clone + std::fmt::Debug {
     /// Account index (aka nonce) type. This stores the number of previous
     /// transactions associated with a sender account.
     type Index: Parameter
@@ -81,28 +81,27 @@ pub trait System {
         + Ord
         + Default;
 
-    /// Converting trait to take a source type and convert to `AccountId`.
-    ///
-    /// Used to define the type and conversion mechanism for referencing
-    /// accounts in transactions. It's perfectly reasonable for this to be an
-    /// identity conversion (with the source type being `AccountId`), but other
-    /// modules (e.g. Indices module) may provide more functional/efficient
-    /// alternatives.
-    type Lookup: StaticLookup<Target = Self::AccountId>;
+    /// The address type. This instead of `<srml_system::Trait::Lookup as StaticLookup>::Source`.
+    type Address: Codec + Clone + PartialEq + MaybeDebug;
 
     /// The block header.
     type Header: Parameter
         + Header<Number = Self::BlockNumber, Hash = Self::Hash>
         + DeserializeOwned;
+}
 
-    /// The aggregated event type of the runtime.
-    type Event: Parameter + Member + From<Event>;
-
-    /// The `SignedExtension` to the basic transaction logic.
-    type SignedExtra: SignedExtension;
-
-    /// Creates the `SignedExtra` from the account nonce.
-    fn extra(nonce: Self::Index) -> Self::SignedExtra;
+/// Blanket impl for using existing runtime types
+impl<T: srml_system::Trait + std::fmt::Debug> System for T
+where
+    <T as srml_system::Trait>::Header: serde::de::DeserializeOwned,
+{
+    type Index = T::Index;
+    type BlockNumber = T::BlockNumber;
+    type Hash = T::Hash;
+    type Hashing = T::Hashing;
+    type AccountId = T::AccountId;
+    type Address = <T::Lookup as StaticLookup>::Source;
+    type Header = T::Header;
 }
 
 /// The System extension trait for the Client.