The short answer
On site, a single-storey extension typically takes 8–12 weeks; a double-storey takes 14–20 weeks. But the full journey from decision to completion — including design, planning and procurement — usually takes 6–12 months for a single-storey and 9–18 months for a double-storey. See planning permission and building regulations for the consent stages that add time.
One of the most common frustrations with extension projects is discovering that the build itself is the shortest part of the process. Design, planning consent and procurement can easily take as long as the construction stage, or longer. Setting realistic expectations at the outset — and building buffer time into your programme — reduces stress and avoids the common mistake of booking a builder before the planning decision has been made.
Extension timeline at a glance
- Design and drawings 4–10 weeks typically
- Planning permission 8–13 weeks for a decision (from validation)
- Permitted development Up to 42 days for prior approval
- Procurement (getting quotes) 2–6 weeks
- Single-storey on site 8–12 weeks typically
- Double-storey on site 14–20 weeks typically
Stage one: design (4–10 weeks)
The design stage starts when you appoint an architect or designer. They will measure up, discuss your brief, prepare initial concept designs, develop these into planning drawings and eventually produce detailed construction drawings for building regulations and the builder. For a single-storey PD extension, this can take as few as four to six weeks. For a project requiring full planning permission, the design needs to be more developed before submission, and the architect may need to resolve planning policy questions or consult the LPA informally before applying — allow six to ten weeks or more. See do I need an architect? for how to choose the right professional.
Stage two: planning and consents (6–16 weeks)
If your extension falls within permitted development, you may need only a prior approval application under the neighbour consultation scheme, which takes up to 42 days (about six weeks). If you need full planning permission, the LPA has an 8-week statutory target for householder applications (13 weeks for larger or more complex applications). In practice, processing times vary and some authorities run slower than their target, particularly for controversial applications or where further information is requested. If the application is refused and you appeal, add six to twelve months. This is the stage that most frequently causes programmes to slip.
| Extension type | Consent type | Typical decision time |
|---|---|---|
| Single-storey (PD, under limits) | Prior approval or none | 42 days or automatic |
| Single-storey (full planning) | Householder application | 8–13 weeks |
| Double-storey | Householder application | 8–13 weeks (often longer) |
| Listed building | Listed building consent + planning | 8 weeks each (in parallel ideally) |
Stage three: procurement (2–6 weeks)
Once you have planning consent and construction drawings, you go out to builders for quotes. Getting at least three written quotes, evaluating them on a like-for-like basis, taking references, and negotiating a contract typically takes two to six weeks. Experienced extension builders in popular areas are often busy: you may need to wait two to four months for a start date. This is normal; it is not a sign that you should rush to the first builder who is available immediately. See how to choose a builder for an extension for the full procurement checklist.
Stage four: build (8–20 weeks on site)
The on-site build time depends primarily on the type and size of extension. A typical single-storey rear extension of 20–30 m² takes 8–12 weeks. A double-storey extension takes 14–20 weeks. Factors that extend the programme include:
- Complex ground conditions: unexpected underground structures, high water table or made ground can delay groundwork.
- Structural discoveries: uncovering unforeseen issues with the existing structure (rotten lintels, inadequate foundations at the junction) adds time.
- Client changes: changing specification or scope mid-build is the single most common cause of delay and cost overrun.
- Lead times: certain products (bespoke glazing, underfloor heating systems, specialist structural steel) have long lead times and must be ordered early.
Stage five: completion and sign-off (1–4 weeks)
The final building control inspection confirms compliance and the completion certificate is issued. This is the formal end of the regulatory process. Allow one to four weeks for snagging, final decorating and the builder completing outstanding items, plus the building control body’s processing time. Building control sign-off is not a formality: inspectors do occasionally require remedial work before issuing the certificate.
This page gives typical timelines for England. Programmes vary by LPA processing times, builder availability and project complexity. Always build contingency into your programme and discuss the realistic timeline with your architect and builder at the outset.
Start your extension project
Getting written quotes from experienced extension builders is the right step once you have a clear brief. No obligation.
Frequently asked questions
How long does planning permission take for an extension?
A householder planning application has an 8-week statutory determination target in England (13 weeks for larger applications). In practice some authorities take longer. Prior approval for PD extensions takes up to 42 days.
How long does it take to build a single-storey extension?
On site, typically 8–12 weeks for a standard single-storey rear or side extension. The full project from first meeting with an architect to receiving the completion certificate is usually 6–10 months.
Can a builder start before building regulations approval?
You can start after serving a building notice or submitting Full Plans (even before formal approval in some cases), but the inspector must be notified before work begins. Starting without any notification is a breach of the regulations.
What is the most common cause of extension delays?
Planning processing times and builder availability are the most common external delays. Client changes to specification mid-build are the most common internal cause. Plan carefully, finalise your design before work starts, and build programme contingency in from the start.
Sources & further reading
- Planning Portal — Householder planning applications: determination targets (England)
- LABC — Building regulations process: notification, inspections and completion certificates
- GOV.UK — Planning permission: how long does it take?
- RICS — Guidance on domestic construction programme management
This is general information about house extensions in England and is not planning, structural, legal or financial advice. Costs, timescales and outcomes vary with your design, ground conditions, specification and local authority. Always obtain written quotes and verify planning and building regulations requirements with your local planning authority before committing to any works.