API Documentation — BILTD Pro

Authentication

All API requests require a Bearer token in the Authorization header. Generate API keys from your BILTD Pro settings.

Request header

Authorization: Bearer biltd_live_xxxxxxxxxxxxxxxxxxxxxxxx

Keep your API keys secure. Do not expose them in client-side code. Rotate keys immediately if compromised.

Rate Limits

Rate limits are enforced per API key on a rolling hourly window.

Plan tier	Lookups/hour	Batch queries/hour
Starter	100	5
Practice	500	20
Office	1,000	50
Enterprise	1,000+	100+

Rate limit status is returned in every response:

Response headers

X-RateLimit-Reset: 2026-04-11T15:00:00.000Z

When a rate limit is exceeded, the API returns HTTP 429 Too Many Requests.

GET/api/v1/design-envelope

Returns the permitted development zones for a residential property: the PD rear extension zone (up to 3m/4m), Prior Approval zone (up to 6m/8m), and curtilage area (outbuilding limit). Derived from HMLR INSPIRE plot boundaries + OS/OSM building footprints + GPDO 2015 rules.

Query parameters

Parameter	Type	Required	Description
`uprn`	string	One of `uprn`, `lat+lng`, or `address`	Unique Property Reference Number. Preferred over lat/lng for accuracy.
`lat` + `lng`	number	One of	WGS84 decimal coordinates. Both required if using lat/lng.
`address`	string	One of	Full UK address string. Used to geocode if UPRN/coordinates unavailable.
`houseType`	string	No	OS Places house type hint: `detached`, `semi-detached`, `terraced`. Improves property type inference.
`includeConstraints`	boolean	No	Set `true` to include designated land overlay (conservation area, AONB, listed building, Article 4). Adds ~1s latency.
`includeFeasibility`	boolean	No	Set `true` to include financial feasibility estimates (build cost range, value uplift). Requires postcode in address string.

Example request

cURL

curl -H "Authorization: Bearer biltd_live_xxx" \
  "https://pro.getbiltd.com/api/v1/design-envelope?uprn=10002004456&includeConstraints=true"

JavaScript

const res = await fetch(
  'https://pro.getbiltd.com/api/v1/design-envelope?uprn=10002004456&includeConstraints=true',
  { headers: { Authorization: 'Bearer biltd_live_xxx' } }
);
const data = await res.json();
console.log(data.zones?.rearExtPdDepthM); // e.g. 4 (metres)

Python

import httpx

r = httpx.get(
    'https://pro.getbiltd.com/api/v1/design-envelope',
    params={'uprn': '10002004456', 'includeConstraints': 'true'},
    headers={'Authorization': 'Bearer biltd_live_xxx'}
)
data = r.json()
print(data['propertyType'])  # 'detached'

Response schema

JSON response

{
  "uprn": "10002004456",
  "lat": 51.5123,
  "lng": -0.1234,
  "address": "1 EXAMPLE STREET, LONDON, SW1A 1AA",
  "plotAreaM2": 320,
  "curtilageM2": 210,
  "propertyType": "semi_detached",
  "titleNumber": "AGL123456",
  "dataSources": {
    "plot": "hmlr_inspire",
    "building": "os_features"
  },
  "constraintsSummary": {      // null if includeConstraints not set
    "hasConstraints": false,
    "allRightsBlocked": false,
    "reducedRights": false,
    "conservationArea": false,
    "aonb": false,
    "nationalPark": false,
    "listedBuilding": false,
    "article4": false,
    "appliedPdDepthM": 3,
    "appliedPaDepthM": 6
  },
  "feasibility": null,         // null if includeFeasibility not set
  "zones": {
    "rearExtPdDepthM": 3,
    "rearExtPaDepthM": 6,
    "pdOutbuildingM2": 105,
    "curtilageM2": 210,
    "propertyType": "semi_detached",
    "rearExtPd": { "type": "Feature", "geometry": { ... } },
    "rearExtPa": { "type": "Feature", "geometry": { ... } },
    "outbuildingArea": { "type": "Feature", "geometry": { ... } },
    "sideExt": null
  },
  "plotPolygon": { "type": "Polygon", "coordinates": [ ... ] },
  "buildingPolygon": { "type": "Polygon", "coordinates": [ ... ] }
}

Error codes

HTTP status	Meaning
`400`	Missing required parameters
`401`	Invalid or missing API key
`404`	No INSPIRE plot polygon for this property (leasehold, unregistered, or new development)
`422`	Could not resolve coordinates from provided address
`429`	Rate limit exceeded
`500`	Internal error — contact support

Use cases

Estate agents & portals

Enrich property listings with “development potential” attributes. Show buyers the maximum rear extension available at a glance.

Mortgage lenders & AVMs

Factor permitted development potential into automated valuations. Properties with significant PD headroom consistently command a premium.

Planning consultants

Automate pre-application feasibility screening across client portfolios. Identify the strongest development opportunities before site visits.

PropTech platforms

Embed plot boundaries and PD zones directly into your map layers. GeoJSON output is compatible with Mapbox, Leaflet, and QGIS.

Get started

Generate an API key from your BILTD Pro account settings. Free trial: 100 lookups on sign-up.

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