The short answer... For a typical single-storey rear or side-return extension in South-West London, allow 8–12 months from your first conversation with an architect to moving back in. Only four months of that is the build itself (with a competent contractor). Add a loft conversion, or take on the whole house, and allow 12–14 months. The rest goes on the survey, design, planning approval, technical drawings, party wall agreements and finding the right builder. That is where most of the waiting happens, and where good planning saves the most time. Knowing the stages up front helps you plan around school terms, finance and where you will live during the works. Below we set out each stage and the timings we typically see.
Stage by stage
1 - Measured survey (2 weeks) Everything starts with an accurate survey of your home as it is now. The property will be measured and the drawings are produced of the existing plans, elevations and sections, which every later drawing is built on. Book it early: it is quick, and nothing else can start without it.
2 - Concept and scheme design (3–4 weeks) This is where your brief becomes a design. We explore overall high level design with 3D visuals and layout options, which we then work with you to develop the preferred scheme. Most of the variation here comes from how quick and clear your feedback is, so a clear brief at the start and quick turnover of feedback after the initial options keeps this stage swift.
3 - Planning permission (8-12 weeks) Councils aim to decide householder applications within eight weeks of validation. Some projects can go ahead under permitted development instead. Even then, we usually recommend a Lawful Development Certificate, which takes a similar time but gives you written proof the work is lawful. Larger single-storey rear extensions may use the prior approval route, which is decided in around six weeks. If the planning application is low risk, we can make good use of that wait: technical design (architectural and structural) and party wall advice can start while the council considers the scheme, so the project keeps moving. Conservation areas, objections from neighbours or errors from the council can add time. A refusal and resubmission typically adds another two to three months, which is why getting the design and planning strategy right before submitting matters so much.
4 - Technical design and building regulations (4-6 weeks) We usually start the detailed construction drawings and specification while the planning application is being decided, so they are ready ahead of approval. This tends to be prepared in tandem with the Structural Engineer’s drawings. We will liaise with them to provide a coordinated overall package, which can be issued to building control. This is also the set of drawings builders will price from, so the more complete it is, the more accurate your quotes will be.
5 - Party wall agreements (4–8 weeks, alongside stages 4 and 6) If you are building on or near a shared wall or digging foundations close to a neighbour's, you will need to serve party wall notices. The notice periods are one or two months depending on the work. If your neighbours consent, it can be quick; if they dissent, surveyors prepare an award, which can take one to two months. Serving notices as soon as planning is approved, and running them alongside technical design and tender, keeps this off the critical path.
6 - Tender and appointing a builder (4 weeks, plus around 4 weeks to start on site) We send the drawings to a shortlist of builders, typically giving them around three weeks to price. For most clients, we then compare the quotes on a like-for-like basis and complete a tender analysis, clarifying any gaps and help you agree a contract with a builder. Allow around four more weeks for the builder to start on site – please note smaller builders may require 3months before starting. See our guide to selecting a builder for works to your home.
7 - The build (4 months for a single-storey extension) A typical rear or side-return extension takes around four months on site, from demolition and groundworks through to the shell, glazing, services and finishes. Adding a loft conversion or refurbishing the rest of the house usually extends this to six to nine months. If the kitchen is being rebuilt, many families move out for some or all of the build. Living in throughout, will usually have the effect of adding circa 1month to the program. Long-lead items such as steel-framed glazing, rooflights and kitchens should be ordered early so they arrive when the builder needs them.
What speeds things up, and what slows them down
Ways to save time
Agree your brief and budget before design starts, so fewer rounds of changes are needed. Send a well-considered mood board ahead of the initial designs.
Check early whether permitted development applies; it can avoid a full planning application.
Approach other consultants as soon as planning is submitted (structural engineer, party wall surveyor and building control).
Consider going ahead with the detailed design whilst you await planning
Serve party wall notices as soon as you have the engineer’s preliminary drawings
Approach builders early to check they are keen for the project ahead of sending the formal tender.
Choose and order long-lead items, such as glazing, rooflights and the kitchen, before the build begins.
Common causes of delay
Conservation areas, listed buildings and Article 4 directions, which mean more scrutiny at planning.
Neighbour objections or a planning refusal that needs a resubmission.
Design changes after planning approval, which may need a fresh application.
Design changes after detailed design completed
Party wall disputes where neighbours dissent and surveyors are needed.
Builder availability, and slow periods around August and Christmas.
Surprises on site, such as poor existing foundations, drains in the wrong place or hidden structural issues.
Client changes during construction
Planning an extension?
We have taken more than 750 projects through this process across London, from the first survey to the final sign-off. You can see how we handle each stage on our Process page, or browse finished extensions in our portfolio.
