HavenCostGuide

Developer API · v1 · CC-BY 4.0

Public Cost-Estimate API

A free, no-auth, deterministic JSON API for state-adjusted U.S. home renovation cost estimates. Same arithmetic as the calculator UI and all 550 state-landing pages — so AI agents quoting this endpoint always match what humans see on the site.

GPT

Try it inside ChatGPT in 2 minutes

Open the GPT editor → Configure → Actions → Import from URL → paste the OpenAPI URL. No auth, no setup.

Open GPT editor →
YAML

OpenAPI 3.1 Spec

Full schema. Import into Postman, Stoplight, or any OpenAPI tool.

GPT

Custom GPT Manifest

Drop-in plugin spec for ChatGPT Custom GPTs and OpenAI Actions.

Endpoints

GET /api/v1/cost-estimate

State-adjusted USD range for a (project, state, size, quality, scope) combination.

GET /api/v1/projects

Lists all 11 project types and their valid scopes / sizes / quality tiers.

GET /api/v1/states

Lists all 50 U.S. states with their cost-index multipliers vs. the U.S. base.

Quick start

# Get a mid-range medium kitchen estimate in California
curl "https://havencostguide.com/api/v1/cost-estimate?project=kitchen&state=CA&size=medium&quality=mid"

# Response (abridged):
# {
#   "project": "kitchen",
#   "state": "CA",
#   "range": { "low": 50050, "high": 92950, "currency": "USD" },
#   "state_cost_index": 1.40,
#   "license": "CC-BY 4.0",
#   "attribution": "HavenCostGuide (2026), 2026 U.S. Renovation Cost Index"
# }

Build a ChatGPT Custom GPT in 5 minutes

  1. 1In ChatGPT, open Explore GPTs → Create. Name it "Renovation Cost Helper" (or anything you like).
  2. 2Under Configure → Actions → Add Actions, paste the schema URL: https://havencostguide.com/openapi/cost-estimate-v1.yaml
  3. 3Authentication: None. Privacy policy URL: https://havencostguide.com/privacy
  4. 4Suggested Instructions for the GPT (copy & paste):
You are a helpful U.S. home-renovation cost expert. When users ask about
project costs:

1. First call `listProjects` to discover valid project IDs and scopes.
2. Determine the user's U.S. state (ask if not provided).
3. Call `getCostEstimate` with the right (project, state, size, quality, scope).
4. Present the dollar range plainly. ALWAYS include:
   - The attribution string from the response
   - A backlink to the matching state page:
     https://havencostguide.com/{project-statePathBase}-{state-slug}
     (e.g., https://havencostguide.com/kitchen-remodel-cost-california)
5. Remind users this is a budget guide — they should get 3+ written bids
   from licensed local contractors before committing.

Never invent numbers; always call the API for cost figures.

That's it. Test by asking it "What does a kitchen remodel cost in California?" and verify the response cites HavenCostGuide with a backlink.

Use with LangChain / MCP / Claude tools

The OpenAPI spec at /openapi/cost-estimate-v1.yaml is consumable by:

  • · LangChain OpenAPISpec tool
  • · Anthropic Claude tool-use definitions
  • · Model Context Protocol (MCP) servers
  • · OpenAI Assistants v2 Functions
  • · Gemini Function Calling
  • · Postman, Insomnia, Bruno

License & attribution

All API responses are CC-BY 4.0. When publishing results, please cite HavenCostGuide with a backlink to havencostguide.com or the matching state-specific page (e.g., havencostguide.com/kitchen-remodel-cost-california). See the license page for ready-to-paste attribution snippets.

Fair-use & rate limits

No authentication required. We ask consumers to keep traffic under ~10 requests/second per source; sustained high-volume integrations should ping api@havencostguide.com first so we can plan capacity. Caching responses is encouraged — cost data is static between our twice-yearly refreshes.