A practical sequence from survey and planning to build and handover
Short answer: a London house extension timeline is not just the number of weeks spent building. Survey, design, planning, party wall work, structural information, building control, quoting and contractor lead-in all come before the site programme. The reliable way to plan is to clear each gate in the right order.
Homeowners often ask how long an extension will take when the project is still an idea. That is understandable, but it mixes several different clocks. A council decision has a different timetable from a structural engineer. A neighbour may need time to respond. A builder cannot price accurately if drawings and the finish specification are still changing.
This guide breaks the journey into practical stages. It does not promise a start or completion date. It shows what needs to be ready before a date can be trusted.
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Stage 1: survey, brief and design
The first useful milestone is not a sketch that looks good. It is a clear brief supported by a measured survey. The design needs to answer how the new room will be used, how it connects to the existing house, where light enters, what happens to drainage and services, and how the structure may work.
At concept stage, options can still move quickly. Once a preferred layout is chosen, the design should become detailed enough for planning, structural input and a realistic quote. If the kitchen, glazing, floor finish or heating strategy remains undecided, the builder can only price assumptions.
Stage 2: planning permission or permitted development
Some extensions can fall within permitted development rights, while others need a planning application. Property type, size, position and local restrictions all matter. Conservation areas, listed buildings, flats and homes with earlier alterations can need a different route.
The standard target for many householder planning applications is eight weeks, but that is a decision target rather than a guarantee. Validation questions, revised drawings or local authority workload can extend the process. The Planning Portal extension guidance is a useful starting point. Our existing guide to planning permission for London extensions explains the main questions in more detail.
Stage 3: structure, building control and party wall work
Structural design turns the layout into something that can be built. Openings, steelwork, foundations and support to the existing house need proper information. Building-control requirements also affect insulation, fire safety, ventilation, drainage and inspections.
Party wall work can run alongside design and approvals, but it should not be left until the builder is ready to start. Neighbour response times are outside the contractor's control. GOV.UK publishes a party wall guide for building owners.
Stage 4: specification, quotes and contractor lead-in
A quote is most useful when every builder is pricing the same information. Drawings alone are not always enough. The scope should cover demolition, structure, first and second fix services, windows and doors, roof construction, plastering, decoration, flooring, kitchen or bathroom allowances, waste, access and site setup.
Good builders also book ahead. A planning decision does not automatically create an immediate start slot. Materials, specialist trades and inspections must be sequenced. Our quote comparison checklist helps you check whether competing documents cover the same work.
Stage 5: the build programme
Access, welfare, temporary protection, service checks and agreed working areas come first.
Foundations, drainage, walls, steels and roof structure create the new shell.
Roof finishes, glazing, doors and external details allow internal work to progress safely.
Electrical, plumbing, heating, insulation and changes to the original house are coordinated.
Second fix, decoration, floors, cabinetry and fittings turn the shell into a usable room.
Final checks, agreed defects and documents should be closed before the project is signed off.
A rear extension with straightforward access may be easier to sequence than a wraparound or double-storey scheme. That does not make any one type automatically quick. Structure, drainage, glazing, finish level and changes inside the original house can matter more than floor area alone.
What commonly stretches the timeline
- changing the layout or specification after structural work has been designed;
- waiting for planning, listed-building or conservation-area decisions;
- late party wall discussions or neighbour access questions;
- drains, poor ground, hidden structure or services discovered after opening up;
- restricted delivery and waste access on London streets;
- long-lead glazing, kitchens, specialist finishes or custom joinery;
- weather during exposed structural and roofing stages.
Not every delay can be prevented. The practical aim is to identify risks early, make decisions before they block work and keep the client informed when the sequence changes.
How BCS plans the work
BCS agrees a project-specific programme once the scope, information and site conditions are clear. During the build, the team keeps clients informed about progress, upcoming decisions and any issue that affects the sequence. That is more useful than offering a generic completion date before the project has been surveyed.
See the difference between side return, rear and wraparound extensions, review what moves a London extension budget, or browse the completed Greenwich house extension.
Ready to build a realistic programme?
Start with the BCS house extension service, browse completed projects, or request a free quote.
