How Long do House Extensions Really Take?

How long do house extensions take on average?

The honest answer is: it depends. Most house extensions across the UK take between nine and fifteen months from first consultation to final sign-off. That covers every stage - design, planning, technical drawings, approvals, and construction. A straightforward single-storey rear extension built under Permitted Development Rights can sometimes be completed in as little as six to nine months, while larger or more complex designs (such as double-storey additions or projects involving full refurbishments) often stretch to twelve to eighteen months. It’s important to remember that the actual “build time” - the period on site - might only be four to six months. The longer portion usually comes from design development, planning applications, building control checks, and party wall matters. Homeowners who start the process early and plan efficiently tend to save the most time. At DeVis Architecture, we break every extension into clear, manageable stages - from the initial survey to the day your new space is ready to enjoy. By understanding the entire process upfront, you’ll know exactly what to expect, when each milestone should happen, and how long your dream home transformation is likely to take.

What stages make up a typical house extension project?

Every extension follows a similar sequence, even if timelines vary. The design and feasibility stage (3–6 weeks) covers your survey, layout ideas, and budget. Next comes planning permission or permitted development approval (8–12 weeks). Once granted, we move into the technical design and building regulations stage (4–8 weeks), preparing detailed drawings and specifications. Then comes pre-construction, when your builder is appointed and materials are ordered (4–6 weeks). Finally, construction takes around 12–26 weeks, depending on size, weather, and site conditions, followed by snagging and sign-off (2–4 weeks). Altogether, that’s roughly nine to fifteen months from start to finish. Knowing each step helps manage expectations and avoid costly downtime - something we guide every DeVis client through from the outset.

What can cause delays to a house extension?

Even well-organised projects can face unexpected delays. The most common are planning hold-ups, as local councils often take longer than the target eight weeks to approve drawings. Party Wall disputes can also add four to eight weeks if neighbours delay responses. Material lead times are another factor - bespoke glazing, steelwork, and joinery can take up to ten weeks to arrive. Add to that weather interruptions in winter, and even small design changes mid-build can easily snowball into further delays. At DeVis Architecture, we help clients anticipate these challenges by ordering early, coordinating consultants closely, and aligning planning with realistic timeframes. That way, projects run as efficiently as possible, without compromising quality.

When is the best time to start a house extension?

If you want to start building in spring or summer, begin the design and planning stages in autumn or early winter. This gives enough time for surveys, drawings, and approvals before ideal construction weather arrives. For most clients, September to January is the perfect window to kick off the process. It means planning decisions are in place by March or April, and work can start once the ground is dry and days are longer. Even for Permitted Development projects, starting early helps you secure a reliable builder and plan materials in advance. At DeVis Architecture, we structure projects around seasonal efficiency - helping homeowners across London and Surrey avoid delays, reduce costs, and complete their extensions on schedule.