Extension and loft timeline calculator
How long your extension or loft conversion takes from first drawings to the builder leaving: design, planning or a Lawful Development Certificate, party wall notices, building control, tender, lead-in, the build and snagging. Pick a start month to see rough dates.
A planning aid, not a programme. Build durations are my own published figures for London projects; planning periods are the statutory targets, and the borough figures come from the government’s planning statistics. Every source is listed under how we worked out these figures.
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Where the time actually goes
Most people picture the build. On a typical London kitchen extension the build is well under half the calendar. The rest is design, planning, the party wall process, getting comparable quotes and waiting for a good builder to be free. Three things make the biggest difference:
- Choosing the right planning route. A scheme that fits permitted development with a Lawful Development Certificate is usually quicker than a full application, and prior approval has a fixed 42-day period. Check yours with the planning permission checker.
- Running things in parallel. Party wall notices, building control and the tender can all run while planning is being decided, or straight after. Done in sequence they can add two or three months.
- Ordering long-lead items early. Large glazing is typically 8 to 12 weeks from order. If it is ordered when the walls go up, the build stops and waits.
For the stage-by-stage detail, read how long an extension or loft conversion takes, how long a kitchen extension really takes and how long planning permission takes in London. To put a cost against the programme, use theextension cost calculator or theloft conversion cost calculator.
Householder planning decisions by borough, year to June 2026
From the government’s PS2 planning statistics for householder developments, 1 July 2025 to 30 June 2026. "Within 8 weeks" excludes decisions made under an agreed extension of time; "in time" counts both.
| Council | Decisions | Granted | Within 8 weeks | Extension of time | In time |
|---|---|---|---|---|---|
| Hammersmith & Fulham | 506 | 97.8% | 30% | 65.4% | 79.8% |
| Kensington & Chelsea | 664 | 93.7% | 38% | 58.7% | 91.9% |
| Wandsworth | 977 | 91.9% | 61.1% | 32.8% | 92.4% |
| Lambeth | 595 | 83.4% | 56% | 40.5% | 93.4% |
| Richmond upon Thames | 1,302 | 83.3% | 70.9% | 28.2% | 98.3% |
| Merton | 759 | 92.9% | 39.9% | 46.5% | 85.2% |
| All London councils | 23,973 | 85.8% | 64.5% | 30.9% | 93.5% |
HPM calculations from the PS2 open data table. Westminster is left out because its figures for early 2026 look incomplete in the published data. The statistics do not give an average decision time, so the calculator uses the 8-week target plus an allowance, not a borough average.
How we worked out these figures
Design (4 to 8 weeks), tender (4 to 8 weeks), the build durations and snagging (2 to 6 weeks) are the figures I publish from running London projects. Planning uses the statutory periods: 8 weeks from validation for a householder application or a Lawful Development Certificate and 42 days for prior approval, with 1 to 3 weeks for validation (a third-party estimate; there is no statutory period). The upper planning figure of 13 weeks is my own allowance for an agreed extension of time, not a statistic. Party wall notice periods (two months, shown as 9 weeks; one month, shown as 5 weeks) and the 5-week full plans decision come from GOV.UK. Tender, building control and the party wall notice run side by side; the build starts when the slowest of them finishes. Neighbour disputes are not modelled because there is no reliable published average for how long an award takes. Checked 6 October 2026.
- HPM — How long does a house extension or loft conversion actually take? (Ben Murrell, 29 July 2026): Design to submission 4–8 weeks; tender 4–8 weeks; build: rear 16–18 weeks, wraparound 20–26, Velux loft 4–6, dormer 10–12, hip-to-gable or mansard 8–12; snagging 2–6 weeks.
- HPM — How long does a kitchen extension really take? (Ben Murrell, 31 July 2026): Single-storey kitchen / side-return build 17–18 weeks; glazing lead time 8–12 weeks from order.
- GOV.UK — District planning application statistics (PS2), last four quarters to 30 June 2026 (open data): Householder developments by local planning authority: decisions, granted, decided within 8 weeks (excluding extension-of-time / performance agreements) and decisions made under an agreement. HPM calculations.
- Planning Portal — Lawful Development Certificate: Proposed use or operation: half the normal planning fee. "Should be decided within 8 weeks", starting once the application is validated.
- legislation.gov.uk — GPDO 2015, Schedule 2, Part 1 (householder permitted development): Class A (extensions): 3 m / 4 m single-storey rear limit, 6 m / 8 m with prior approval (not on article 2(3) land), 4 m height, 3 m eaves within 2 m of a boundary, side extensions single storey and no wider than half the original house, two-storey 3 m deep and 7 m from the rear boundary, A.2 conservation area exclusions. Class B (roof enlargements): 40 m³ terrace / 50 m³ other, nothing beyond the front roof slope, not on article 2(3) land.
- Architectural Drawings UK — How long does planning take in London (2026): Third-party estimate: "Validation can add 1–3 weeks before the clock starts." Validation has no statutory period.
- GOV.UK — The Party Wall etc. Act 1996: explanatory booklet: Notice at least two months before work to an existing party wall; one month before excavating near a neighbour’s foundations or building a new wall on the boundary. No reply within 14 days = a dispute, and surveyors are appointed.
- GOV.UK — Building regulations approval: how to apply: Full plans: decision within 5 weeks, or 2 months with your consent. Building notice: start 2 days after submitting. Completion certificate usually within 8 weeks of completion.
Two-storey extensions, basements and whole-house renovations are not built into the calculator because I have not published standard durations for them; choose "something else" and enter your builder’s programme.
Common questions
How long does a house extension take from start to finish in London?
For a single-storey rear or side-return extension that needs a planning application, allow roughly 37 to 54 weeks (about 8½ to 12½ months) from first drawings to the end of the build: 4 to 8 weeks of design, 9 to 16 weeks for validation and a decision, 4 to 8 weeks to tender while party wall notices run, a builder lead-in (the calculator assumes 4 weeks until you enter your builder’s figure), then 16 to 18 weeks on site. Snagging adds 2 to 6 weeks. Permitted development schemes are quicker because the planning stage is shorter.
How long does planning permission take for an extension?
The statutory target for a householder application is 8 weeks from validation, and validation itself can add 1 to 3 weeks. In the year to June 2026, the share of householder decisions made within 8 weeks without an agreed extension of time ranged from 30% in Hammersmith & Fulham to 71% in Richmond, so it is worth allowing extra time.
Can I serve party wall notices before planning is approved?
Yes. The Party Wall Act notice periods (two months for work to a party wall, one month for excavation) are not tied to planning, so serving notice while the application is being decided means the notice period has often run by the time you have a builder. The notice is only valid for a year, so do not serve it too early.
How long does a loft conversion take to build?
On my own published figures: about 4 to 6 weeks for a Velux or rooflight conversion, 10 to 12 weeks for a rear dormer, and 8 to 12 weeks for a hip-to-gable or mansard, once the builder is on site. Add design, any planning or certificate, party wall notices and tender time before that.
What usually delays an extension timeline?
In my experience: a planning application that needs an extension of time, party wall neighbours who appoint their own surveyor, structural steel and glazing lead times (8 to 12 weeks from order for large glazing), and changes made after the build starts. Ordering long-lead items early and keeping decisions ahead of the builder protects the programme.
Before you start: planning permission checker. Costs: extension cost calculator · loft conversion cost calculator · renovation fees calculator · all resources.
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Phone 07525 474 849 or send me your drawings. No fee for the first conversation.
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