Building or extending a home is a sequence of decisions, most of which become expensive to reverse after a specific point. This guide walks through the stages in order, explains which decisions are locked at each one, and is honest about where budgets and programmes actually go wrong.
Stage 1 — Feasibility and budget
Before drawings, establish two numbers: what the finished property will be worth, and what you can genuinely spend. Build costs are usually expressed per square metre, and in 2026 a good-quality UK extension or new build sits in a wide band depending on region, specification and access. Add professional fees, VAT where applicable, and a contingency of 10–15% that you do not treat as spending money.
Stage 2 — Planning permission and permitted development
Many extensions fall under permitted development and need no full application, but the limits on height, depth and proportion are precise, and listed buildings and conservation areas remove them entirely. A lawful development certificate is worth having even when you are confident — it is the document a buyer's solicitor will ask for years later.
Where a full application is required, allow eight to thirteen weeks for determination, plus time to respond to consultation comments.
Stage 3 — Design and structural engineering
This is where money is saved or lost. Structural calculations determine steel sizes, foundation depth and how much of the existing building must be altered. A design that respects existing loadbearing lines will always be cheaper to build than one that fights them — and that decision is made on paper, not on site.
Stage 4 — Building regulations and party wall
Building control approval covers structure, fire safety, insulation, ventilation, drainage and electrical safety. It runs alongside planning, not instead of it. If you are building on or near a shared boundary, the Party Wall Act requires notice to your neighbours — typically two months for wall work, one month for excavation. Serving notice late is one of the most common causes of a delayed start.
Stage 5 — Tender and contract
Get three quotations against the same drawings and specification. A quote that is far below the others is not a bargain; it is usually a different scope. Insist on a written contract covering payment stages, programme, variation procedure and retention. Never pay a large deposit for materials that have not been delivered.
Stage 6 — Groundworks and substructure
Excavation, foundations, drainage connections and the damp proof course. Ground conditions are the single largest source of genuine, unavoidable cost overrun — trees, old foundations, soft ground and services in unexpected places. This is what your contingency exists for.
Stage 7 — Superstructure and weathertight
Walls, steels, floor structure, roof, windows and doors. Reaching weathertight is the psychological halfway point and a sensible payment milestone. From here, weather stops driving the programme.
Stage 8 — First fix
Plumbing, electrics and heating pipework installed before walls are closed. Every decision about socket positions, lighting, radiator locations and bathroom layouts must be final here. Changes after plastering cost several times what they would have cost now.
Stage 9 — Plaster, second fix and finishing
Plastering, then flooring, kitchens, sanitaryware, doors, decoration. Allow drying time between plaster and decoration; rushing it is how new houses end up with cracked, patchy walls in their first winter.
Stage 10 — Completion and handover
Building control final inspection, electrical and gas certificates, warranties, and a snagging list agreed in writing with a date for completion. Hold a retention — commonly 2.5–5% for six months — and do not release it until snags are genuinely done.
Where projects actually go wrong
- Starting on site before the design is finished, so decisions get made under time pressure.
- No contingency, so the first ground surprise becomes a crisis.
- Changes after first fix, each one cheap in isolation and ruinous in aggregate.
- Choosing the lowest quote without comparing scope.
- No written variation process, so cost arguments happen at the end rather than as they arise.
None of these are technical problems. They are process problems, and they are the reason we insist on a fixed specification and a written programme before the first tool arrives.
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