How Long Does a House Extension Take in London? A Stage-by-Stage Timeline
A typical single-storey house extension in London often takes several months on site, but the whole journey from first idea to handover is usually longer, because design, approvals and neighbour notices come first. The figures below are indicative only. Your own timeline will depend on size, complexity, planning route, weather and how quickly decisions are made.
Before you build: design and approvals
Stage 1: Briefing and design
Before anything is built you need drawings. This stage covers the brief, a measured survey, concept layouts and developed drawings. It can take a few weeks to a couple of months, depending on how quickly you settle on a layout and how many revisions you want. Design and build firms can compress this stage because design and construction teams work together.
Stage 2: Planning or permitted development
If your extension needs a planning application, a householder application usually has a statutory determination period of around eight weeks once the council has validated it, though some take longer if more information is requested. If you plan to rely on permitted development, you may want a Lawful Development Certificate, which also takes some weeks. Check with your local council for current times.
Stage 3: Building regulations and structural design
An engineer will design the steelwork and foundations, and building control will check the drawings or oversee the work. This can run alongside planning, and may take a few weeks.
Stage 4: Party Wall process
If neighbours are affected, notices need to be served ahead of the works, typically with a minimum of one or two months' notice depending on the type. If a dispute arises and surveyors are appointed, it can take longer. Our guide to the Party Wall Act is worth reading early.
Stage 5: Pre-construction
Once approvals are in place, there is the selection of finishes, ordering long-lead items such as windows, bi-fold doors or steel, and agreeing the contract and start date. Lead times for glazing in particular can run to several weeks, so ordering early helps.
The build itself
A typical sequence on site, with indicative durations for a modest single-storey rear extension, looks like this:
Set-up and demolition: site set-up, scaffold, removal of the existing back wall or structure.
Groundworks and foundations: excavation, drainage, concrete and inspection.
Structure: walls, steel beams and roof.
Weathertight: roof covering, windows and doors, so the interior can dry out.
First fix: electrics, plumbing, heating, insulation.
Plastering and drying time.
Second fix and finishes: kitchen fitting, flooring, decorating, final connections.
Snagging and sign-off.
For a modest single-storey extension, many projects fall somewhere around three to five months on site, with larger, double-storey or wraparound schemes taking longer. These are broad ranges, not promises.
What commonly causes delay
Late changes to the design or specification.
Slow decisions on finishes or kitchen layout.
Unexpected ground conditions or hidden drainage.
Waiting for approvals or inspections.
Long lead times for bespoke items.
Bad weather during groundworks and roofing.
Good planning reduces the chances. Locking down your specification before you start, ordering early and keeping a clear line of communication with your builder make a real difference. Our extensions service page explains how we manage projects from start to finish.
Seasons matter too. Groundworks in a wet winter can be slower, and some finishes need dry, mild conditions to cure well. None of this prevents winter building, but a good programme allows for it.
It also pays to be realistic about snagging. After the main work finishes, expect a short period of tidying up, adjustments and final certificates. Your builder should walk through the project with you, note anything outstanding and agree a date to complete it. Do not move furniture back or book a house-warming until building control has signed off and the final cleaning is done.
If you want a firmer timeline, ask your builder for a written programme with milestones, such as foundations complete, roof weathertight and plastering finished. Reviewing it together every couple of weeks makes slippage visible early, when it is still easy to fix, and it helps you plan around deliveries, inspections and your own decisions on finishes.
Do not forget your own part in the programme. Decisions on kitchens, tiles, sanitaryware and paint colours often hold up the job more than the weather does, so make them in good time and keep a shared list with your builder.
Frequently asked questions
Can an extension be built faster?
Sometimes, by overlapping design with approvals and ordering early, but quality and inspection stages should not be rushed.
Will I be without a kitchen?
Often yes for part of the build, depending on the layout. Plan a temporary kitchen set-up in advance.
When should I contact a builder?
Early. A builder can advise on buildability, costs and sequencing before the drawings are finalised.
Start your timeline with TNK
We can map out the stages for your property and give you an honest view on timing. Use the cost calculator to gauge budget and then get in touch to book a conversation.
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