How long does a house extension or renovation take, start to finish?

People ask how long the building work takes. The build is usually the shorter half. From first idea to finished room, most extension projects in the UK run 9-18 months, and the paperwork is what stretches them.

A realistic timeline for a single-storey rear extension

• Brief and appointing a designer: 2-4 weeks.
• Measured survey and concept design: 3-6 weeks.
• Planning application or Lawful Development Certificate: 8-12 weeks from submission, plus 2-3 weeks to prepare it.
• Technical design and structural engineering: 4-8 weeks, can partly overlap with planning at risk.
• Building regulations approval: 5-8 weeks, or via building notice on simpler jobs.
• Party wall notices and Award: 8-12 weeks, run in parallel from planning submission.
• Tendering and appointing a contractor: 4-8 weeks.
• Contractor lead-in before starting on site: 4-12 weeks for a good builder — the good ones are booked.
• Construction: 12-20 weeks for a typical single-storey rear extension.
• Snagging and completion: 2-4 weeks.

Loft conversions typically run 8-12 weeks on site. Two-storey extensions 16-26 weeks. A whole-house renovation, 6-12 months on site.

What the build weeks actually look like

Set-up and demolition, foundations and drainage, brick and blockwork to damp course, superstructure and steels, roof structure and covering, windows and doors, first fix services, plastering, second fix, decoration, external works. Foundations and roof watertight are the two milestones that matter most — after watertight, weather stops driving the programme.

What causes delay

• Late decisions. A kitchen layout undecided at first fix stops electrics.
• Variations. Every change re-prices and re-sequences.
• Ground conditions. Deeper foundations near trees or old drains add weeks.
• Long-lead items. Structural steel, bespoke glazing, sliding doors and specialist bricks can run 8-16 weeks. Order at technical design stage, not when the opening is ready.
• Statutory sign-off. Building control inspections must happen before work is covered up.
• Weather and holidays. Christmas typically removes two weeks; a wet January slows groundworks.

How to shorten it honestly

• Overlap what can legitimately overlap: party wall notices during planning, structural design during determination.
• Fix the specification before you tender. Decisions made once, early, are free.
• Order long-lead items as soon as the design is technically fixed.
• Appoint a contractor with a real start date, in writing, and accept that a builder available next week is often available for a reason.
• Keep a contingency of 15-20% on the programme, not just the budget.

Living through it

Decide early whether you are staying. Staying saves rent but slows the build — protection, restricted hours and daily reinstatement all cost time. Moving out for the noisy phase alone is a common compromise.

Next step

BuildWise Workspace generates a programme for your specific project from your answers about planning route, property type and complexity, then shows it as a schedule and Gantt chart, with dependencies so you can see exactly what a two-week delay to planning does to your finish date.