Your team and the process
How long does a house extension take, from first idea to moving in?
Short answer
Most of the time goes before building starts. The law fixes the main decision periods in England: 8 weeks for a householder planning application or a lawful development certificate, 42 days for prior approval of a larger home extension, and 5 weeks for a Building Regulations full plans application, or longer if you agree it in writing. Party wall notices must be served one or two months before the work they cover.
For the rest there is no statutory figure. HomeBuilding's guide, published 18 October 2024, quotes builders and designers giving 2 to 3 months for design, at least 3 months to allow for planning, a month for tender and another for the builder to start, and 6 to 16 weeks on site for a single-storey extension.
The statutory periods
| Step | Period | Where it comes from |
|---|---|---|
| Householder planning application | 8 weeks, beginning the day after the council receives the application, or a longer period agreed in writing | Development Management Procedure Order 2015, article 34(2)(b) and (c) |
| Lawful development certificate | 8 weeks, or a longer period agreed in writing | Development Management Procedure Order 2015, article 39(10) |
| Prior approval, larger home extension (Class A) | 42 days. Neighbours are given at least 21 days to comment. If the council has not decided within 42 days, the work may go ahead | General Permitted Development Order 2015, Schedule 2, Part 1, paragraph A.4(5) and (10) |
| Prior approval, additional storey (Class AA) | No deemed approval. Work must not begin until the council's written prior approval is received | General Permitted Development Order 2015, Schedule 2, Part 1, paragraph AA.3(13) |
| Building Regulations full plans | 5 weeks, or a longer period agreed in writing | Building Regulations 2010, regulation 14A(6) |
| Party wall notice, new wall on the boundary line | At least one month before work starts | Party Wall etc. Act 1996, section 1 |
| Party wall notice, work to a party wall | At least two months before work starts | Party Wall etc. Act 1996, section 3 |
| Party wall notice, excavation near a neighbour | At least one month before work starts | Party Wall etc. Act 1996, section 6 |
The development must not begin before the occurrence of one of the following (a) the receipt by the developer from the local planning authority of a written notice that their prior approval is not required; (b) the receipt by the developer from the local planning authority of a written notice giving their prior approval; or (c) the expiry of 42 days following the date on which the information referred to in sub-paragraph (2) was received by the local planning authority without the local planning authority notifying the developer as to whether prior approval is given or refused.
Two points are often got wrong. The 5-week Building Regulations period has had no two-month cap in England since 1 October 2023, when section 16 of the Building Act 1984 was omitted; the current rule is regulation 14A(6), and some older guidance still quotes the two months. And if building control misses the 5 weeks, regulation 14A(7) says the failure is not to be treated as a grant or a rejection, so there is no approval by default. The party wall periods, and when a notice becomes a dispute, are in party wall notice periods.
The published figures for the rest
These are from one national guide, HomeBuilding's "How long does an extension take?", published 18 October 2024, which quotes builders and designers. They are not Parallel's figures.
| Stage | Published figure |
|---|---|
| Design, to a full set of drawings for planning | At least 2 months; 2 to 3 months for new or complex designs |
| Planning | Allow at least 3 months |
| Tender | 1 month, then another for the builder to start |
| Build, single-storey without a kitchen | Around 6 to 8 weeks |
| Build, single-storey with a kitchen and internal alterations | 8 to 16 weeks |
| Build, two-storey without internal work | About 8 to 12 weeks |
| Build, with bathrooms or significant internal alterations | Six months or more |
| A simpler single or double extension, start to finish | About 6 to 12 months, with 3 to 4 months on site |
Part of the gap between that allowance and the 8 statutory weeks is that the period runs from receipt of a complete application with its fee, and that amendments and agreed extensions add time.
How the stages overlap
The stages do not have to run end to end. Technical design and the structural engineer's calculations can proceed during the planning decision period, at the risk of rework if the council asks for changes. The tender can be prepared alongside the full plans application. Party wall notices can go out once the design that affects the wall is fixed, so that the notice period has run before the builder is ready to start. The order of the stages, and what each produces, is set out in the RIBA Plan of Work 2020 stages for a home project.
What adds time
- A planning refusal or a request for amendments.
- Conditions that must be discharged before work starts.
- Listed building consent, where the house is listed, alongside any planning application.
- A neighbour dispute under the Party Wall etc. Act 1996.
- The builder's start date, for which HomeBuilding's guide allows a month after the tender.
- Decisions left late during the build. The Federation of Master Builders' budgeting guide (23 February 2026) notes that delayed decisions can lead to rushed substitutions and delivery delays.
Our own typical durations by project type will be added here.
Sources
- Town and Country Planning (Development Management Procedure) (England) Order 2015, articles 34 and 39 legislation.gov.uk
- General Permitted Development Order 2015, Schedule 2, Part 1, paragraphs A.4 and AA.3 legislation.gov.uk
- Building Regulations 2010, regulation 14A legislation.gov.uk
- Building Act 1984, section 16, omitted in England from 1 October 2023 legislation.gov.uk
- Party Wall etc. Act 1996, sections 1, 3 and 6 legislation.gov.uk
- HomeBuilding: How long does an extension take?, 18 October 2024 homebuilding.co.uk
- Federation of Master Builders: How to budget for a renovation, 23 February 2026 fmb.org.uk