diff --git a/src/builder/README.md b/src/builder/README.md
new file mode 100644
index 000000000..fa721abb8
--- /dev/null
+++ b/src/builder/README.md
@@ -0,0 +1,7 @@
+# Builder JSON-RPC API
+
+The Builder JSON-RPC API is a collection of methods that external block building software must 
+implement. This interface allows the communication between the consensus layer and external builders.
+
+This API is in *active development* and currently [specified as a markdown document](./specification.md).
+A schema will follow once the specification stabilizes.
diff --git a/src/builder/specification.md b/src/builder/specification.md
new file mode 100644
index 000000000..e5a4923f2
--- /dev/null
+++ b/src/builder/specification.md
@@ -0,0 +1,316 @@
+# Builder API
+
+This document specifies the Builder API methods that the Consensus Layer uses to interact with external block builders.
+
+## Structures
+
+### `ExecutionPayloadV1`
+
+Mirror of [`ExecutionPayloadV1`][execution-payload].
+
+### `ExecutionPayloadHeaderV1`
+Equivalent to `ExecutionPayloadV1`, except `transactions` is replaced with `transactionsRoot`.
+- `parentHash`: `DATA`, 32 Bytes
+- `feeRecipient`:  `DATA`, 20 Bytes
+- `stateRoot`: `DATA`, 32 Bytes
+- `receiptsRoot`: `DATA`, 32 Bytes
+- `logsBloom`: `DATA`, 256 Bytes
+- `prevRandao`: `DATA`, 32 Bytes
+- `blockNumber`: `QUANTITY`, 64 Bits
+- `gasLimit`: `QUANTITY`, 64 Bits
+- `gasUsed`: `QUANTITY`, 64 Bits
+- `timestamp`: `QUANTITY`, 64 Bits
+- `extraData`: `DATA`, 0 to 32 Bytes
+- `baseFeePerGas`: `QUANTITY`, 256 Bits
+- `blockHash`: `DATA`, 32 Bytes
+- `transactionsRoot`: `DATA`, 32 Bytes
+
+### `BlindedBeaconBlockV1`
+- `slot`: `QUANTITY`, 64 Bits
+- `proposerIndex`: `QUANTITY`, 64 Bits
+- `parentRoot`: `DATA`, 32 Bytes
+- `stateRoot`: `DATA`, 32 Bytes
+- `body`: `object`, [`BlindedBeaconBlockBodyV1`](#blindedbeaconblockbodyv1)
+
+### `BlindedBeaconBlockBodyV1`
+- `randaoReveal`: `DATA`, 96 Bytes
+- `eth1Data`: `object`, [`Eth1DataV1`](#eth1datav1)
+- `graffiti`: `DATA`, 32 Bytes
+- `proposerSlashings`: `Array`, [`ProposerSlashingV1`](#proposerslashingv1)
+- `attesterSlashings`: `Array`, [`AttesterSlashingV1`](#attesterslashingv1)
+- `attestations`: `Array`, [`AttestationV1`](#attestationv1)
+- `deposits`: `Array`, [`DespositV1`](#depositv1)
+- `voluntaryExits`: `Array`, [`SignedVoluntaryExitV1`](#signedvoluntaryexitv1)
+- `syncAggregate`: `object`, [`SyncAggregateV1`](#syncaggregatev1)
+- `executionPayloadHeader`: `object`, [`ExecutionPayloadHeaderV1`](#executionpayloadheaderv1)
+
+### `Eth1DataV1`
+- `depositRoot`: `DATA`, 32 Bytes
+- `depositCount`: `QUANTITY`, 64 Bits
+- `blockHash`: `DATA`, 32 Bytes
+
+### `ProposerSlashingV1`
+- `signedHeader1`: `object`, [`SignedBeaconBlockHeaderV1`](#signedbeaconblockheaderv1)
+- `signedHeader2`: `object`, [`SignedBeaconBlockHeaderV1`](#signedbeaconblockheaderv1)
+
+### `BeaconBlockHeaderV1`
+- `slot`: `QUANTITY`, 64 Bits
+- `proposerIndex`: `QUANTITY`, 64 Bits
+- `parentRoot`: `DATA`, 32 Bytes
+- `stateRoot`: `DATA`, 32 Bytes
+- `bodyRoot`: `DATA`, 32 Bytes
+
+### `AttesterSlashingV1`
+- `attestation1`: `object`, [`IndexedAttestationV1`](#indexedattestationv1)
+- `attestation2`: `object`, [`IndexedAttestationV1`](#indexedattestationv1)
+
+### `IndexedAttestationV1`
+- `attestingIndices`: `Array`, `QUANTITY`, 64 Bits
+- `data`: `object`, [`AttestationDataV1`](#attestationdatav1)
+- `signature`: `DATA`, 96 Bytes
+
+### `AttestationDataV1`
+- `slot`: `QUANTITY`, 64 Bits
+- `index`: `QUANTITY`, 64 Bits
+- `beaconBlockRoot`, `DATA`, 32 Bytes
+- `source`: `object`, [`CheckpointV1`](#checkpointv1)
+- `target`: `object`, [`CheckpointV1`](#checkpointv1)
+
+### `CheckpointV1`
+- `epoch`: `QUANTITY`, 64 Bits
+- `root`: `DATA`, 32 Bytes
+
+### `AttestationV1`
+- `aggregationBits`: `DATA`, 0 to 256 Bytes
+- `data`: `object`, [`AttestationDataV1`](#attestationdatav1)
+- `signature`: `DATA`, 96 Bytes
+
+### `DepositV1`
+- `proof`: `Array`, 32 Bytes
+- `data`: `object`, [`DepositDataV1`](#depositdatav1)
+
+### `DepositDataV1`
+- `pubkey`: `DATA`, 48 Bytes
+- `withdrawalCredentials`: `DATA`, 32 Bytes
+- `amount`: `QUANTITY`, 64 Bits
+- `signature`: `DATA`, 96 Bytes
+
+### `VoluntaryExitV1`
+- `epoch`: `QUANTITY`, 64 Bits
+- `validatorIndex`: `QUANTITY`, 64 Bits
+
+### `SyncAggregateV1`
+- `syncCommitteeBits`: `DATA`, 64 Bytes
+- `syncCommitteeSignature`: `DATA`, 96 Bytes
+
+#### Signed Containers
+
+### `SignedBeaconBlockHeaderV1`
+- `message`: `object`, [`BeaconBlockHeader`](#beaconblockheaderv1)
+- `signature`: `DATA`, 96 Bytes
+
+### `SignedVoluntaryExitV1`
+- `message`: `object`, [`VoluntaryExitV1`](#voluntaryexitv1)
+- `signature`: `DATA`, 96 Bytes
+
+#### SSZ Objects
+
+Consider the following definitions supplementary to the definitions in [`consensus-specs`][consensus-specs].
+
+##### `ValidatorRegistrationV1`
+
+```python
+class ValidatorRegistrationV1(Container):
+    feeRecipient: Bytes20
+    gasLimit: uint64
+    timestamp: uint64
+    pubkey: BLSPubkey
+```
+
+##### `BuilderBidV1`
+
+```python
+class BuilderBidV1(Container):
+    header: ExecutionPayloadHeader
+    value: uint256
+    pubkey: BLSPubkey
+```
+
+###### `BlindedBeaconBlock`
+
+```python
+class BlindedBeaconBlock(Container):
+    slot: Slot
+    proposer_index: ValidatorIndex
+    parent_root: Root
+    state_root: Root
+    body: BlindedBeaconBlockBody
+```
+
+###### `BlindedBeaconBlockBody`
+
+```python
+class BlindedBeaconBlockBody(Container):
+    randao_reveal: BLSSignature
+    eth1_data: Eth1Data
+    graffiti: Bytes32
+    proposer_slashings: List[ProposerSlashing, MAX_PROPOSER_SLASHINGS]
+    attester_slashings: List[AttesterSlashing, MAX_ATTESTER_SLASHINGS]
+    attestations: List[Attestation, MAX_ATTESTATIONS]
+    deposits: List[Deposit, MAX_DEPOSITS]
+    voluntary_exits: List[SignedVoluntaryExit, MAX_VOLUNTARY_EXITS]
+    sync_aggregate: SyncAggregate
+    execution_payload_header: ExecutionPayloadHeader
+```
+
+## Errors
+
+The list of error codes introduced by this specification can be found below.
+
+| Code | Message | Meaning |
+| - | - | - |
+| -32000 | Server error | Generic client error while processing request. |
+| -32001 | Unknown hash | No block with the provided hash is known. |
+| -32002 | Unknown validator | Unknown validator. |
+| -32003 | Unknown fee recipient | No known mapping between validator and fee recipient. |
+| -32004 | Unknown block | Block does not match the provided header. |
+| -32005 | Invalid signature | Provided signature is invalid. |
+| -32006 | Invalid timestamp | Provided timestamp was invalid. |
+| -32600 | Invalid request | The JSON sent is not a valid Request object. |
+| -32601 | Method not found | The method does not exist / is not available. |
+| -32602 | Invalid params | Invalid method parameter(s). |
+| -32603 | Internal error | Internal JSON-RPC error. |
+| -32700 | Parse error | Invalid JSON was received by the server. |
+
+Each error returns a `null` `data` value, except `-32000` which returns the `data` object with a `err` member that explains the error encountered.
+
+For example:
+
+```
+$ curl https://localhost:8550 \
+    -X POST \
+    -H "Content-Type: application/json" \
+    -d '{"jsonrpc":"2.0","method":"engine_getPayloadV1","params": ["0x1"],"id":1}'
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "error": {
+    "code": -32000,
+    "message": "Server error",
+    "data": {
+        "err": "Database corrupted"
+    }
+  }
+}
+```
+
+## Routines
+
+### Signing
+
+All signature operations should follow the [standard BLS operations][bls] interface defined in `consensus-specs`.
+
+There are two types of data to sign over in the Builder API:
+* In-protocol messages, e.g. [`BlindedBeaconBlock`](#blindedbeaconblock), which should compute the signing root using [`compute_signing_root`][compute-signing-root] and use the domain specified for beacon block proposals.
+* Builder API messages, e.g. [`builder_registerValidatorV1`](#builder_registerValidatorV1) and the response to [`builder_getHeader`](#response-2), which should compute the signing root using [`compute_signing_root`][compute-signing-root] and the domain `DomainType('0xXXXXXXXX')` (TODO: get a proper domain).
+
+As `compute_signing_root` takes `SSZObject` as input, client software should convert in-protocol messages to their SSZ representation to compute the signing root and Builder API messages to the SSZ representations defined [above](#sszobjects).
+
+## Methods
+
+### `builder_status`
+
+#### Request
+
+- method: `builder_status`
+- params: `null`
+
+#### Response
+
+- result: `enum`, `"OK" | null`
+- error: code and message set in case the builder is not operating normally.
+
+#### Specification
+
+1. Builder software **MUST** return `-32000: Server error` if it is unable to respond to requests. `err` **SHOULD** be set explaining the issue and `result` **MUST** be `null`.
+
+### `builder_registerValidatorV1`
+
+#### Request
+
+- method: `builder_registerValidatorV1`
+- params:
+  1. `message`: `object`
+      1. `feeRecipient`: `DATA`, 20 Bytes - Address of account which should receive fees.
+      2. `gasLimit`: `QUANTITY`, 64 bits - Target gas limit for block.
+      3. `timestamp`: `QUANTITY`, 64 bits - Unix timestamp of announcement.
+      4. `pubkey`: `DATA`, 48 Bytes - BLS public key of validator.
+  2. `signature`: `DATA`, 96 Bytes - BLS signature over [`ValidatorRegistrationV1`](#validatorregistrationv1).
+
+#### Response
+
+- result: `enum`, `"OK" | null`
+- error: code and message set in case an exception happens while registering the validator.
+
+#### Specification
+1. Builder software **MUST** verify `pubkey` corresponds to an active or pending validator, otherwise return error `-32002: Unknown validator`.
+2. Builder software **MUST** verify `signature` is valid under `pubkey`, otherwise return error `-32005: Invalid Signature`.
+3. Builder software **MUST** respond to requests where `timestamp` is less than or equal to the latest announcement from the validator or more than 10 seconds in the future with error `-32007: Invalid timestamp`.
+4. Builder software **MUST** return `result` as `"OK"` if the request succeeds, `null` otherwise.
+
+### `builder_getHeaderV1`
+
+#### Request
+
+- method: `builder_getHeaderV1`
+- params:
+  1. `slot`: `QUANTITY`, 64 Bits - Slot number of the block proposal.
+  2. `pubkey`: `DATA`, 48 Bytes - BLS public key of validator.
+  3. `parentHash`: `DATA`, 32 Bytes - Hash of execution layer block the proposer will use as the proposal's parent.
+
+#### Response
+
+- result: `object`
+    - `message`: `object`
+        - `header`: [`ExecutionPayloadHeaderV1`](#executionpayloadheaderv1).
+        - `value`: `QUANTITY`, 256 Bits - Payment in wei that will be paid to the `feeRecipient` account.
+        - `pubkey`: `DATA`, 48 Bytes - BLS public key associated with the builder.
+    - `signature`: `DATA`, 96 Bytes - BLS signature over [`BuilderBidV1`](#builderbidv1).
+- error: code and message set in case an exception happens while getting the header.
+
+#### Specification
+1. Builder software **SHOULD** respond immediately with the `header` that increases the `feeRecipient`'s balance by the most.
+2. Builder software **MUST** return a `header` whose `parentHash` matches the request's `parentHash`. If `parentHash` is not known by the builder, it **MUST** return `-32001: Unknown hash`.
+3. Builder software **MUST** return `-32002: Unknown validator` if `pubkey` does not map to the validator that is expected to propose at `slot`.
+4. Builder software **MUST** return `-32003: Unknown fee recipient` if the builder does not have a `feeRecipient` mapped to the validator.
+5. Builder software **MAY** set the `feeRecipient` for the block to a different address than the address mapped to the validator so long as a payment equal to `value` is made to `feeRecipient`.
+6. Builder software **MUST** return a header whose `gasLimit` is equal to the validator's registered `gasLimit` or as close as possible under the constraints of the consensus rules.
+
+### `builder_getPayloadV1`
+
+#### Request
+
+- method: `builder_getPayloadV1`
+- params:
+  1. `message`: [`BlindedBeaconBlock`](#blindedbeaconblockv1).
+  2. `signature`: `DATA`, 96 Bytes - BLS signature over [`BlindedBeaconBlock`](#blindedbeaconblockv1).
+
+#### Response
+
+- result: [`ExecutionPayloadV1`](#executionpayloadv1).
+- error: code and message set in case an exception happens while proposing the payload.
+
+#### Specification
+1. Builder software **MUST** be able to un-blind the [`ExecutionPayloadHeaderV1`](#executionpayloadheaderv1) in `message`, otherwise the return `-32004: Unknown block`.
+2. Builder software **MUST** verify that `signature` is a BLS signature over `block` using [`verify_block_signature`][verify-block-signature] from the validator that is expected to propose in the slot. If the signature is determined to be invalid or from a different validator than expected, the builder **MUST** return `-32005: Invalid signature`. It's possible that competing chains could have different proposer shuffling, causing multiple proposer indexes to be valid for a given slot.
+
+[consensus-specs]: https://github.com/ethereum/consensus-specs
+[bls]: https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/beacon-chain.md#bls-signatures
+[compute-signing-root]: https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/beacon-chain.md#compute_signing_root
+[bytes20]: https://github.com/ethereum/consensus-specs/blob/dev/ssz/simple-serialize.md#aliases
+[uint256]: https://github.com/ethereum/consensus-specs/blob/dev/ssz/simple-serialize.md#basic-types
+[execution-payload-header]: https://github.com/ethereum/consensus-specs/blob/dev/specs/bellatrix/beacon-chain.md#executionpayloadheader
+[execution-payload]: https://github.com/ethereum/execution-apis/blob/main/src/engine/specification.md#executionpayloadv1
+[hash-tree-root]: https://github.com/ethereum/consensus-specs/blob/dev/ssz/simple-serialize.md#merkleization
+[beacon-block]: https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/beacon-chain.md#beaconblock
+[verify-block-signature]: https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/beacon-chain.md#beacon-chain-state-transition-function
diff --git a/wordlist.txt b/wordlist.txt
index e9e86cb1d..c1790db6d 100644
--- a/wordlist.txt
+++ b/wordlist.txt
@@ -1,5 +1,6 @@
 apis
 attributesv
+bls
 bytecode
 configurationv
 eip
@@ -21,9 +22,10 @@ rlp
 rpc
 schemas
 secp
+sha
+ssz
 statev
 statusv
-sha
 uint
 updatedv
 url