10 Common Mistakes Homeowners Make When Building an Extension

**Mistake 1 — Choosing the lowest quote without understanding what's excluded** The lowest quote is almost never the cheapest project. When three contractors quote for the same job and the quotes vary by 25–40%, the difference is almost never all profit margin — it is usually scope. The cheapest quote has excluded something: drainage, structural engineer's fees, Building Control applications, scaffolding, first-fix electrics beyond a ring main, or an allowance for unforeseen conditions.

How to avoid it: Ask for a fully itemised breakdown of what is included. List every element you expect — drainage, steelwork, scaffolding, Building Control fees, provisional sums for unforeseen work — and ask each contractor to confirm explicitly whether each item is included or excluded. Only then can you compare like with like.

**Mistake 2 — Starting on site before drawings are ready** Pressure to start the project — often driven by the contractor's programme or the homeowner's desire to see progress — leads some clients to authorise groundworks before construction drawings, structural engineer's calculations, or Building Regulations approval are in place. The result: the contractor builds to their own assumptions, discovers the specification needs to change once the proper drawings are available, and raises expensive variation claims.

How to avoid it: Never authorise foundation or structural works until construction drawings are approved by Building Control (full plans route) or submitted (building notice route) and structural engineer's drawings are issued. Planning permission is not the trigger for starting construction — Building Regulations approval and construction drawings are.

**Mistake 3 — Underestimating the cost of finishes and fittings** Contractors typically price the shell and structure of an extension — the fabric, M&E first fix, and sometimes plasterwork. Kitchens, bathrooms, flooring, doors, ironmongery, electrical fittings, lighting, and furniture are not in most contractor scopes. Homeowners are frequently surprised that after paying the contractor's final invoice, there is another £20,000–£60,000 of finishes and furnishing to spend.

How to avoid it: At the outset of the project, produce a full project budget including both the contractor's scope and the client-supplied elements. Work backwards from what you can afford to a scope the contractor can deliver, leaving the necessary budget for finishes.

**Mistake 4 — Not sorting the Party Wall Agreement before starting** Party wall notice must be served on adjoining owners at least 2 months (for Section 2 party structure works) or 1 month (for Section 6 excavation works) before work commences. Homeowners who discover this obligation after the project has already started must either delay or risk enforcement by an adjoining owner. Retrospectively regularising Party Wall obligations is possible but costly and stressful.

How to avoid it: Confirm the party wall position with the architect or contractor before signing the contract. Serve notice at least 2 months before the intended start date for structural works.

**Mistake 5 — Failing to attend Building Control stage inspections** Building Control sends an inspector to sign off specific stages of construction — foundations before concrete poured, drainage before backfilled, steelwork before hidden behind walls, insulation before plasterboard applied. If these stages are missed (because the contractor didn't notify Building Control, or poured concrete before the inspector arrived), Building Control may require opening up works to inspect — expensive, disruptive, and entirely avoidable.

How to avoid it: Confirm with the contractor that they will notify Building Control at every required stage. Ask to see the Building Control application and the inspection schedule. On a well-managed project, the contractor's site manager handles this automatically — but verify.

**Mistake 6 — Allowing verbal changes without written confirmation** Programme pressure leads to on-site decision-making: the homeowner says 'can you move that window 200mm to the left?' and the foreman says 'fine, no problem.' This generates a variation to the contract — which has cost and programme implications. Without written confirmation of the change and its cost impact, the variation is disputed at final account stage.

How to avoid it: Any change to the agreed scope, even apparently minor, should be confirmed in writing before it is done, with an agreed cost impact. On a JCT contract, this is a written instruction from the contract administrator. On a simpler arrangement, an email confirmation from the homeowner and a written price from the contractor is sufficient.

**Mistake 7 — Making too many changes during construction** Related to Mistake 6, but broader. Changes during construction are significantly more expensive than changes made at design stage. Moving a wall that has already been built costs demolition + rebuild + making good. Changing the kitchen layout after first-fix plumbing has been run to the original layout means rerouting pipework at full day-rate cost. Design changes made at the drawing stage cost the architect's time — a fraction of the equivalent cost on site.

How to avoid it: Invest in design. Spend time resolving kitchen, bathroom, and room layouts at the drawing stage — before the contractor has built anything. Resist the temptation to 'see how it looks and decide then.'

**Mistake 8 — Paying the final sum before a proper snagging exercise** The snagging process — a systematic walk-through of the completed works, identifying defects, incomplete items, and items not to specification — is the homeowner's main protection at handover. Some homeowners pay the final invoice promptly and in full, only to find the contractor becomes unavailable or slow to respond to snagging items once they have been paid.

How to avoid it: The final payment should be released only against a signed snagging list that either (a) is clear — no outstanding items — or (b) contains only agreed minor items with an agreed retention to cover them. A standard retention of 3–5% of the contract value held until snagging is complete is the professional mechanism. Do not pay the final certificate without a Building Control completion certificate being in hand or imminent.

• **Mistake 9 — Not getting the Building Control completion certificate**
• A Building Control completion certificate is the formal legal record that the works were built to the Building Regulations standards. Without it:
• •The property does not have a legal completion certificate for the works
• •Estate agents and buyers' solicitors will ask for it at the point of sale — if you cannot produce it, you need either a retrospective inspection (which Building Control can refuse if the works are complete and hidden) or indemnity insurance
• •Building Control may have observed a defect at an inspection stage and not issued the certificate until it is rectified — and the homeowner may not know this

How to avoid it: Make issue of the Building Control completion certificate a condition of final payment. Chase your contractor and Building Control until it is in your hands.

**Mistake 10 — Assuming the extension is ready to sell immediately** Newly constructed extensions take time to settle thermally and structurally. Typical issues in the first year: minor cracking at the junction between new and old structure as differential movement occurs; moisture from the new concrete and plaster slowly driving off (new builds typically take 1–2 heating seasons to fully dry out). Attempting to decorate immediately after completion — before the building has dried out — leads to paint failure, cracking, and re-decoration cost.

How to avoid it: Allow 3–6 months after the project completes before final decoration. Apply 'breathing' or emulsion paints in the first season — not oil-based or vinyl silk — to allow the moisture to drive off without being trapped behind an impermeable film.