How Long Does a London Extension Take?

The short answer

Most London house extensions take from several months to well over a year from first idea to completion, once design, approvals and construction are added together. The statutory planning period alone is around eight weeks, and larger, listed or conservation-area projects take longer. A realistic stage-by-stage view is below.

• Feasibility and brief — a few weeks
• Measured survey — around two weeks
• Concept design — several weeks
• Planning application — about 8 weeks statutory, often longer in practice
• Building regulations and structural design — several weeks
• Tender and appointing a builder — a few weeks, plus contractor lead-in
• Construction — a few months for a single-storey rear extension; longer for larger works

Feasibility

A project usually begins with feasibility — establishing what the property and planning context realistically allow and agreeing a brief. This stage can take a few weeks, depending on how quickly decisions are made and whether early advice from the local authority is sought.

Measured survey

Accurate information about the existing building is needed before design develops. A measured survey is usually arranged early and turned around within a couple of weeks, giving a reliable base for the drawings.

Concept design

Developing the design — testing layouts, massing and the relationship to the garden and neighbours — typically takes several weeks, with time for the client to review and refine options. Getting this stage right reduces problems later.

Planning application

Once submitted and validated, a householder planning application has a statutory target determination period of around eight weeks, though in practice it can take longer, particularly for sensitive or contested proposals or where an extension of time is agreed. Conservation-area and listed building applications can take longer still. Pre-application advice, if used, adds time before submission but can reduce risk.

Building regulations

Building regulations drawings and structural engineering are usually prepared after, or alongside, planning. Preparing the technical package and obtaining building control approval typically takes several weeks, depending on the route chosen and the complexity of the work.

Tender

Obtaining and comparing builder quotes — the tender stage — usually takes a few weeks, allowing time for contractors to price and raise queries and for you to review returns and appoint a builder.

Contractor mobilisation

Good builders are often booked ahead, so there can be a wait between appointing a contractor and starting on site. Party wall procedures, where they apply, also need to be completed before relevant works begin and should be started in good time.

Construction

Construction duration depends on the scope: a single-storey rear extension might take a few months, while a larger extension with internal works, a basement or a whole-house refurbishment can take considerably longer. Your contractor will provide a programme for the specific works.

Common delays

Frequent causes of delay include slow decision-making, changes to the brief mid-project, planning conditions that must be discharged before work starts, party wall negotiations, unforeseen conditions in older buildings, and the lead time for appointing a good contractor. Building in realistic time for approvals and procurement avoids disappointment.