Agentic UK Construction Industry Scheme (CIS) subcontractor verification, monthly return filing, and HMRC penalty response service for small main contractors (1–20 staff) who must verify every subcontractor before payment and file CIS300 returns by the 19th of each month. Replaces the bookkeeper or accountant who charges £100–£350 per month to run verifications, calculate deductions, and file returns — a process that triggers automatic £100–£3,000 penalties when late.
Why now
No live signals provided today. Justified by HMRC's ongoing CIS compliance campaign and the well-documented pain in r/Construction and r/smallbusiness threads where contractors describe missing the 19th deadline and receiving automatic penalties — a persistent, high-frequency operational pain point with no agentic incumbent.
Commercial value
HMRC estimates ~150,000 active CIS contractor businesses in the UK. Monthly recurring service with an obvious budget line: bookkeeper fees of £100–£350/month or in-house admin time. ACV: £600–£1,800/year per contractor. TAM: even 1% penetration of active CIS contractors = £9M–£27M ARR. Penalty fear (£100/month automatic, scaling to £3,000) creates strong urgency.
Go-to-market
Reach contractors via Federation of Master Builders member emails, Checkatrade/Rated People contractor forums, and Google Ads on 'CIS return filing service'. Hook: 'Never miss the 19th again — £49/month, we file your CIS300.' First 10 customers via direct DM to contractors complaining about CIS penalties on trade forums. Monthly subscription £49–£149.
2-week MVP
Single workflow: contractor uploads their subcontractor payment spreadsheet (or pastes values into a form), LLM validates it against CIS rules, calculates the correct deduction rate per verified subcontractor, and produces a pre-filled CIS300 return ready for HMRC Online Services submission. Actual HMRC API integration is cut — customer submits the output themselves. Day-1 outcome: contractor receives a verified, correctly calculated CIS300 in under 10 minutes for £49, avoiding a £100 late-filing penalty.
Agent score
0.62 — Commercial score is strong: monthly recurring pain, clear penalty-avoidance budget, large addressable base of 150k contractors. Speed is high because the MVP is a document-in/document-out LLM workflow with no API integration required, and trade forums give direct access to the first 10 customers. Defensibility is low — CIS filing is well-understood and replicable; moat requires building HMRC API integration and accountant referral network over time.
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