How Long Does a House Extension Take? UK Timeline Guide

Ask a builder how long an extension takes and you’ll get a frustratingly vague answer: "it depends." And while that’s technically true, you deserve better than that. Here are realistic timelines based on what actually happens on site, including the planning and preparation that most people forget to factor in.

The Planning and Preparation Stage: 8–13 Weeks

Before any building work starts, there’s a significant preparation phase. This is the stage most homeowners underestimate, and it’s often the reason people get frustrated — they expect to be building by now, but they’re still waiting for paperwork.

• Architectural design: 2–4 weeks for initial designs and revisions

• Planning permission application: BCP Council has 8 weeks to determine a standard householder application. If they request amendments, add another 2–4 weeks

• Building regulations submission: can run parallel to planning, 2–4 weeks for approval of detailed plans

• Structural engineer: 1–2 weeks for calculations

• Party wall notices: if your extension is within 3 metres of a neighbour’s boundary (or 6 metres depending on foundation depth), you must serve notice. If they dissent, appointing surveyors adds 4–8 weeks

Total pre-build time is typically 3–4 months. You can overlap some of these stages, but planning permission is the bottleneck — you can’t start building until it’s granted (unless the extension falls under permitted development).

Single-Storey Extension: 12–16 Weeks Build Time

A typical single-storey rear extension in Dorset follows this rough schedule:

• Weeks 1–2: Groundwork. Dig foundations, pour concrete, install drainage. This is weather-dependent — you can’t dig or pour concrete in heavy rain or frost.

• Weeks 2–4: Blockwork. Build the external walls up to wall plate level (where the roof sits). Cavities filled with insulation.

• Weeks 4–5: Roof structure. Timber or steel frame, felt, battens, and tiling or flat roof membrane.

• Weeks 5–6: Weathertight. Windows and external doors fitted, roof finished, external walls rendered or faced.

• Weeks 6–8: First fix. Electrics, plumbing, and heating roughed in. Insulation fitted. Internal stud walls built.

• Weeks 8–9: The knock-through. The existing external wall between the house and extension is removed or opened up (with RSJ if it’s load-bearing). This is the most disruptive day of the whole project.

• Weeks 9–12: Plastering, second fix electrics and plumbing, tiling, kitchen fitting (if applicable).

• Weeks 12–14: Decoration, flooring, snagging.

Double-Storey Extension: 16–24 Weeks Build Time

A double-storey extension adds roughly 50–70% more build time compared to a single storey. The foundations need to be deeper and wider, there’s twice the blockwork, more structural steel, and a second floor to construct. Scaffolding stays up longer and there’s more internal finishing work.

The additional complexity also means more building regulations inspections and potentially more structural engineering input.

What Causes Delays?

In my experience, these are the most common reasons extensions run over schedule:

Weather

Foundation work is the most weather-sensitive stage. Heavy rain floods the trenches. Frost prevents concrete from curing properly. A bad week at the start can push the whole programme back. This is why starting between March and September is ideal.

Ground Conditions

You don’t know what’s underground until you dig. Tree roots, clay shrinkage, high water tables, old drains, or even buried rubble from previous work can all require deeper or wider foundations than planned. This is one of the most common reasons costs increase too.

Material Lead Times

Most standard materials are readily available, but anything bespoke — specific bricks to match your house, bi-fold doors, specialist glazing, structural steel — needs ordering in advance. Steel beams currently have 2–4 week lead times. Bi-fold doors can be 4–6 weeks.

Client Changes

Changing the kitchen layout once the plumbing is roughed in. Moving a window after the blockwork is up. These mid-build changes are the single biggest cause of delays (and budget overruns). Finalise everything on paper before work starts.

Utility Connections

If your extension needs a new gas, water, or electrical connection, the utility companies work to their own timeline — and it’s rarely fast. Budget 4–8 weeks for utility work and book it early.

How to Keep Your Extension on Schedule

• Choose your builder carefully. A good builder plans the programme in detail before starting and manages subcontractors proactively. Ask to see a project schedule.

• Make decisions early. Kitchen, bathrooms, tiles, flooring — choose all of this before work starts, not when the plasterer is waiting.

• Start at the right time of year. March to May is ideal for getting foundations in during dry, warm weather.

• Book long-lead items immediately. Bi-folds, structural steel, and matching bricks should be ordered as soon as planning is approved.

Planning an extension and want a realistic timeline for your project? We’ll walk through your plans, flag any potential complications, and give you a schedule you can actually rely on.