From first sketch to finished room, a straightforward London extension typically runs six to nine months. Roughly half of that is approvals and procurement rather than building — and the approvals half is the part you can start today.
Last checked against current guidance on 2026-08-09. Rules differ by borough — the local pages below say how.
| Stage | Typical duration | Runs in parallel with |
|---|---|---|
| Measured survey | 1–2 weeks from instruction | — |
| Design and planning drawings | 2–3 weeks | — |
| Council determination | 8 weeks from validation (42 days for prior approval) | Building regulations design |
| Building regulations package | 2–4 weeks to produce | Council determination |
| Structural calculations | 1–2 weeks | Building regulations package |
| Party wall notices | 1–2 months' notice, plus award if dissented | Council determination |
| Tender and builder selection | 3–4 weeks | Building regulations approval |
| Build (single-storey rear extension) | 12–16 weeks | — |
Add the column of things that run in parallel and the honest total for a simple scheme is six to nine months, most of it before the first brick.
Validation, not determination. An application with missing or inaccurate drawings sits in the queue before the eight-week clock even starts.
Councils validate against a published local list — site plan at a recognised scale, existing and proposed elevations, correct fee, ownership certificate. Anything missing goes back, and the eight weeks restart from the day it is complete.
The second common delay is a survey that was never done: a design drawn from an estate agent's floor plan meets the real building at construction stage, and the redesign lands mid-programme.
Council determination is the long pole in this programme, and nothing shortens it except starting earlier with a complete submission. The estimator below scopes the drawing package your project needs, so you know what stage one involves.
Move the controls for an indicative figure. It anchors your budget — it is not a quote. Every project is priced after a free survey.
Build costs are separate and quoted by your contractor. See our published fees.
What you should not start is anything a planning change would waste — bespoke joinery, glazing orders, or a full tender on drawings that may move.
A single-storey rear extension of typical London size is commonly twelve to sixteen weeks on site, longer where the ground is poor, access is through the house, or the scheme includes a large structural opening.
Loft conversions run to a similar timescale; wraparounds and two-storey extensions longer, because there is more structure and more weathering to sequence.
You can amend and resubmit — usually free within twelve months on the same site — or appeal, which for householder cases uses the faster householder appeal service.
Resubmission is almost always quicker than appeal, and the officer's report tells you exactly what to change. Appeals are worth it where the refusal turns on a point of principle rather than a fixable design detail.
Either way the delay is measured in months, which is the strongest argument for resolving daylight, roofline and neighbour issues on the drawing board.
Eight weeks from validation for a householder application, and 42 days for prior approval on a larger home extension. Validation itself can add one to two weeks.
Work can start under a building notice with 48 hours' notice, but on anything structural a full plans application checked in advance is the safer route.
Good London builders are commonly booked three to six months out. Start the conversation during determination, not after approval.
Councils aim for eight weeks on both. The certificate is decided on law and fact rather than planning merit, so a complete, accurate submission matters more than a persuasive one.
An accurate measured survey at the start. Almost every mid-project delay traces back to the building not matching the drawing.
Most of the waiting on an extension is statutory rather than discretionary — the determination period, the consultation window and the building control decision are all fixed in regulation. Where a Local Plan requires pre-application engagement, or CIL is chargeable on the additional floor area, add that to the front of the programme:
