Home extension disputes — what goes wrong and how to prove it
The short answer: extensions fail in predictable ways — foundations, weatherproofing (roof valleys, flashings, thresholds), building-control sign-off, and scope drift on a "fixed" price. Each failure maps to a Consumer Rights Act 2015 breach, and each is provable with the right evidence.
REVIEWED BY: Sean McNamara on 2026-09-03
The six classic failure points
1. Foundations — depth/thickness below the agreed spec or below what ground conditions need; cracking follows (walls, floors, lintels). 2. Weatherproofing — flat-roof falls wrong, valley gutters without proper flashing, thresholds without DPM → water ingress, often seasonal. Photograph during and after rain. 3. Building control — statutory inspections (foundations, drainage, damp-proofing, structure, insulation) must be booked and passed. No sign-off certificate = your house may be hard to sell — a real, quantifiable loss. 4. Structure — steel beams (RSJs) undersized or wrongly installed; openings moved without calculation. 5. Insulation and finishes — cold rooms, cracked plaster over poor backing, skim over unprimed board. 6. The overrun spiral — "found something unexpected", price up, weeks late. See builder walked off the job and quote vs estimate.
Evidence that wins an extension dispute
- The signed quote/specification (the contract) — itemised scope is your measuring stick.
- Building-control records: your inspection sign-offs (or the gaps). You can request the record from your local authority building control (or approved inspector).
- Dated photos: foundations before pour, DPC/tanking before cover, roof layers before felt/batts, and every defect as it appears. If you didn't take stage photos, all is not lost — see below.
- An independent surveyor's report — for structural/waterproofing disputes this is usually the decisive document: find a chartered surveyor via RICS.
- Two or three written remedial quotes to price the schedule of loss.
The loss you might be missing
Beyond repair costs, extension disputes carry: diminution (the house is worth less with an uncertified/defective extension — surveyor can opine), delay costs (renting while the kitchen is unusable), and completion costs (finishing someone else's partial work costs more than doing it whole — document the differential).
Route
1. Letter before action with defects schedule attached. 2. Report via Citizens Advice → Trading Standards (unsafe structures matter to them). 3. Small claims if within the limit; bigger claims need advice (check home insurance legal expenses cover first).
This page is general information, not legal advice. Verified 2026-09-03.
Reviewed by Sean McNamara · 2026-09-03 · Independent information directory — not legal advice.