UK Stamp Duty Land Tax (SDLT) calculator with first-time buyer relief and surcharges

[
  {
    "scheme": "exact",
    "network": "eip155:8453",
    "payTo": "0xFFc458dB291b4ABcE020fE3de4f91F2770E537b1",
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "amount": "1000",
    "maxTimeoutSeconds": 300
  }
]

{
  "bazaar": {
    "info": {
      "input": {
        "body": {
          "is_additional_property": false,
          "is_first_time_buyer": false,
          "is_non_resident": false,
          "price": 350000
        },
        "bodyType": "json",
        "method": "POST",
        "type": "http"
      },
      "output": {
        "example": {
          "breakdown": [
            {
              "band": "£0 – £125,000",
              "rate": "0.0%",
              "tax": 0
            },
            {
              "band": "£125,000 – £250,000",
              "rate": "2.0%",
              "tax": 2500
            },
            {
              "band": "£250,000 – £350,000",
              "rate": "5.0%",
              "tax": 5000
            }
          ],
          "buyer_type": "Standard",
          "effective_rate": 2.14,
          "price": 350000,
          "total_tax": 7500
        },
        "type": "json"
      }
    },
    "schema": {
      "$schema": "https://json-schema.org/draft/2020-12/schema",
      "properties": {
        "input": {
          "additionalProperties": false,
          "properties": {
            "body": {
              "properties": {
                "is_additional_property": {
                  "description": "Second homes / buy-to-let pay a 5% surcharge (Oct 2024 rates).",
                  "type": "boolean"
                },
                "is_first_time_buyer": {
                  "description": "First-time buyers get relief on properties up to £625,000.",
                  "type": "boolean"
                },
                "is_non_resident": {
                  "description": "Non-UK residents pay an extra 2% surcharge.",
                  "type": "boolean"
                },
                "price": {
                  "description": "Property purchase price in GBP.",
                  "minimum": 0,
                  "type": "number"
                }
              },
              "required": [
                "price"
              ],
              "type": "object"
            },
            "bodyType": {
              "enum": [
                "json",
                "form-data",
                "text"
              ],
              "type": "string"
            },
            "method": {
              "enum": [
                "POST",
                "PUT",
                "PATCH"
              ],
              "type": "string"
            },
            "type": {
              "const": "http",
              "type": "string"
            }
          },
          "required": [
            "type",
            "method",
            "bodyType",
            "body"
          ],
          "type": "object"
        },
        "output": {
          "properties": {
            "example": {
              "properties": {
                "breakdown": {
                  "description": "Band-by-band breakdown.",
                  "type": "array"
                },
                "buyer_type": {
                  "type": "string"
                },
                "effective_rate": {
                  "description": "Effective tax rate as a percentage.",
                  "type": "number"
                },
                "price": {
                  "type": "number"
                },
                "total_tax": {
                  "description": "Total SDLT payable in GBP.",
                  "type": "number"
                }
              },
              "type": "object"
            },
            "type": {
              "type": "string"
            }
          },
          "required": [
            "type"
          ],
          "type": "object"
        }
      },
      "required": [
        "input"
      ],
      "type": "object"
    }
  }
}

curl "https://web-production-18a32.up.railway.app/stamp-duty"
# -> 402 Payment Required, accepts[] lists how to pay
# retry with a PAYMENT-SIGNATURE (or PAYMENT header) once paid

const res = await fetch("https://web-production-18a32.up.railway.app/stamp-duty");
if (res.status === 402) {
  const { accepts } = await res.json();
  // pay one of accepts[] via an x402 client, then retry with the payment header
}

import httpx
res = httpx.get("https://web-production-18a32.up.railway.app/stamp-duty")
if res.status_code == 402:
    accepts = res.json()["accepts"]
    # pay one of accepts[] via an x402 client, then retry with the payment header