How Long Does a House Extension Take From Start to Finish?

When people ask how long does an extension take, they usually mean the building work, but that is only a fraction of the real answer. The bricks-and-mortar phase of a single-storey extension is often just ten to fourteen weeks; the whole journey from first idea to a finished, signed-off room is far longer once you add design, drawings, approvals and the inevitable waiting. This guide sets out a realistic UK timeline stage by stage, so you can plan around it instead of being caught out halfway through.

The short answer

• Single-storey extension: roughly 6 to 9 months from first enquiry to completion, of which around 10 to 14 weeks is the actual build.
• Double-storey extension: typically 10 to 14 months in total, with a build phase of around 16 to 22 weeks.

The build itself is the visible, predictable part. Most of the extra months are eaten by design and the approval stages that come before anyone lifts a spade.

Stage by stage

1. Design and drawings (roughly 8 to 12 weeks)

This is where you settle the brief, appoint a designer or architect, and produce the drawings you will need for planning and building control. It sounds quick but often is not: measured surveys, revisions and decision-making all take time, and it is the stage homeowners most often underestimate. Deciding late here is one of the biggest causes of overall delay. Our guide on whether you need an architect helps you scope this.

2. Planning permission or permitted development (up to 8 weeks, often longer)

Many extensions fall under permitted development and skip a full application, which saves weeks. If you do need planning permission, the council has a statutory eight-week target, but in practice many take 10 to 12 weeks, and longer in conservation areas or where neighbours object. Check first whether you even need it using our do you need planning permission guide and permitted development rights explained.

3. Building regulations approval (roughly 5 to 8 weeks)

Separate from planning, building control checks that the work is structurally sound and compliant. This can run alongside the tail end of design, but allow five to eight weeks. See building regulations vs planning permission for how the two differ.

4. Party wall and pre-start admin (2 to 8 weeks)

If your extension affects a shared wall or boundary, you must serve party wall notices, and neighbours have time to respond. This can add weeks, occasionally months if it becomes a dispute, so start it early. Our how to serve a party wall notice guide walks through it.

5. The build (single-storey 10 to 14 weeks, double-storey 16 to 22 weeks)

Only now does construction begin. A rough single-storey sequence looks like:

• Weeks 1 to 3: groundwork and foundations.
• Weeks 3 to 6: walls up to roof level (superstructure).
• Weeks 5 to 8: roof structure, covering, windows and doors, making the shell watertight.
• Weeks 7 to 12: first fix electrics and plumbing, plastering, then second fix.
• Weeks 11 to 14: finishes, decoration, flooring, snagging and building control sign-off.

Double-storey work follows the same stages but takes longer at each. Weather, the state of your existing house, and any structural surprises can all move these dates.

What causes the biggest delays

Most overruns come from a handful of predictable culprits:

• Planning objections and officer queries stretching the approval stage.
• Party wall disputes with neighbours.
• Late design decisions made mid-build, which stall the trades and cause rework.
• Slow material or specialist lead times, such as bespoke glazing or steels.
• Weather and hidden site problems, from bad ground to unexpected drains.

The single best defence is finalising every decision, on layout, kitchen, glazing and finishes, before work starts. Changing your mind on site is where budgets and timelines both slip. Our hidden extension costs guide covers the money side of the same problem.

Planning around the timeline

Because a single-storey extension realistically spans two-thirds of a year, and a double-storey a year or more, start early and be honest about the disruption. If the work affects your kitchen, expect to be without one for part of the build. Get the design locked, approvals in hand and a clear contract with your builder before committing to a start date. For choosing that builder well, see how to choose a builder, and for the official rules, the Planning Portal is the authoritative UK source.

Frequently asked questions

How long does a single-storey extension take? From first enquiry to completion, a single-storey extension typically takes 6 to 9 months. Of that, the actual building work is usually only 10 to 14 weeks; the rest is design, drawings, planning or permitted development checks, and building regulations approval before construction can begin.

How long does the building work itself take? For a single-storey rear extension, the on-site build is generally 10 to 14 weeks, covering foundations, walls, roof, first and second fix, and finishes. A double-storey extension takes longer, usually 16 to 22 weeks on site, because each stage involves more work.

How long does planning permission take for an extension? Councils have a statutory eight-week target for householder applications, but many take 10 to 12 weeks in practice, and longer in conservation areas or where neighbours object. Many extensions avoid this stage entirely if they fall under permitted development, so check that first.

Why do extensions take longer than expected? The most common causes are planning objections and officer queries, party wall disputes, late design changes made during the build, long lead times on materials like glazing or steel, and hidden site problems such as poor ground or unexpected drains. Locking all decisions before work starts prevents most of these.

Can I speed up my extension timeline? Yes, mainly by front-loading. Finalise the design and every finish before starting, confirm whether you need planning permission early, serve party wall notices in good time, and order long-lead items ahead of the build. Delays almost always come from decisions and approvals, not from the building work itself.

How long does a double-storey extension take? A double-storey extension usually takes 10 to 14 months from start to finish, with a build phase of around 16 to 22 weeks. The extra time over a single-storey comes from more structural work, more trades and, often, a full planning application rather than permitted development.