House Extension Timeline: What to Expect from Enquiry to Completion

One of the most common questions homeowners ask before starting an extension project is: how long will this take? It is also one of the most commonly underestimated answers in the industry.

The honest answer is that a rear or side extension in London typically takes between nine and eighteen months from your first conversation to the day you move into the new space — and often longer when planning complications arise. Understanding why helps you plan more effectively and avoid the frustration that comes from unrealistic expectations.

Here is the full journey, with realistic timeframes at each stage.

Stage 1: Initial Enquiry and Brief (Weeks 1–2)

Before any professional is appointed, there is a period of clarifying what you actually want to achieve. This involves conversations with your contractor and architect about the brief, the budget range, what planning context applies, and whether there are any immediate constraints — such as a party wall situation or a protected tree nearby. Getting this stage right reduces costly changes later.

Stage 2: Architect Appointment and Concept Drawings (Weeks 2–10)

Once an architect is appointed, the first phase of design work begins. This covers measured surveys of the existing property, initial concept layouts, and the development of planning drawings. Depending on the architect's workload and the complexity of the brief, this phase typically takes four to eight weeks. Do not rush this stage — poor drawings produce poor planning applications and poor pricing.

Stage 3: Planning Application Preparation and Submission (Weeks 10–14)

Preparing a planning application — planning drawings, design and access statement, application form, and supporting documents — typically takes two to four weeks after the design is agreed. The application is then submitted to the Local Planning Authority.

Stage 4: Planning Decision (Weeks 14–27+)

The LPA has a target of eight weeks to determine a householder application, but in London this is frequently exceeded. Twelve to thirteen weeks is more common in many boroughs. Complex applications or those requiring additional information can take longer still. Budget for thirteen weeks and treat eight as a best case.

For proposals that fall within permitted development, a Lawful Development Certificate can be applied for instead. The LPA has eight weeks to issue this and the process is typically more predictable than a planning application.

Stage 5: Technical Design and Structural Engineering (Post-Planning — Weeks 4–8)

Once planning is approved, the drawings need to be developed to a level of technical detail suitable for Building Regulations and for accurate pricing. This involves structural engineering calculations and drawings, and any specialist inputs such as drainage surveys, energy calculations, or arborist reports where trees are involved. Four to eight weeks is realistic depending on complexity and the responsiveness of the professionals involved.

Stage 6: Party Wall Agreement Where Applicable (Minimum 2 Months' Notice)

If your extension is within three or six metres of a neighbour's foundations, or involves work to a shared wall, the Party Wall etc. Act 1996 applies. Statutory notice must be served at least two months before works start. If neighbours appoint their own surveyor, the process can extend further. Starting this process early — ideally before planning is granted — prevents unnecessary delays.

Stage 7: Pre-Construction Pack and Procurement (Weeks 2–4)

Before site starts, RCB finalises the pre-construction pack: confirmed scope of works, specifications, procurement schedule, and build programme. Material lead times — particularly structural steel, windows, and doors — must be planned well in advance of the start date.

Stage 8: Construction (Weeks 12–20 Depending on Size)

A typical single-storey rear extension runs through groundworks, foundation, structure, first fix, second fix, and completion in twelve to twenty weeks. Double-storey and more complex schemes take longer.

Total Realistic Timeline: 9–18 Months

Most clients are surprised. The build itself is often the shortest phase. The main contributors to programme overrun are: waiting for planning decisions, party wall disputes arising from late notices, incomplete information at the design stage, and finishes decisions being made after the build starts.

What you can do: appoint your contractor and architect early, get drawings completed promptly, and commit to finish selections before site begins. Every decision that is made late on a live site creates cost and programme risk.

RCB Design & Build helps clients manage this process from the first conversation through to completion.

Call: +44 7359 872594 Email: contact@rcbgroup.co.uk