Register for a Companies House API key at developer.companieshouse.gov.uk; authenticate by passing the API key as the HTTP Basic auth username with an empty password.
Search for companies via GET /search/companies?q=<name>&items_per_page=10; each result includes a company_number which is the primary identifier for all subsequent calls.
Fetch full company profile including registered address, SIC codes, accounts dates, and status via GET /company/{company_number}.
Retrieve filed documents (accounts, confirmation statements, officer appointments) via GET /company/{company_number}/filing-history; use the links.document_metadata field to fetch and download individual document PDFs.
Fetch current officers (directors, secretaries) via GET /company/{company_number}/officers and persons with significant control via GET /company/{company_number}/persons-with-significant-control.
Known gotchas
The API rate limit is 600 requests per 5-minute window per API key; large-scale enrichment jobs must throttle requests or use the bulk data products instead.
UK company numbers are zero-padded strings (e.g., '00000006'), not integers; storing them as numbers causes leading-zero loss and lookup failures.
Dissolved company data remains accessible but is clearly flagged; verify company_status before relying on data for active-entity due diligence.
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