An extension programme needs to cover everything between choosing a layout and using the finished rooms. The council's decision period is only one part. Design changes, technical information, builder availability and long-lead items can move the start date even after planning permission is granted.
For initial planning, the worked programme below allows roughly 8–10 months for a straightforward single-storey project. It is an illustrative sequence, not a measured London average or a promise. It assumes a settled brief, no refusal, straightforward ground conditions, timely decisions and an available contractor. A two-storey extension, major refurbishment or difficult site needs its own programme.
An example programme
| Stage | Illustrative allowance | What must be ready for the next step |
|---|---|---|
| Brief, survey and planning design | Weeks 1–6 | An agreed layout and complete application information |
| Council application stage | Weeks 7–16 | The applicable decision and review of its conditions |
| Technical design and tender | Weeks 13–22 | Coordinated construction information, comparable prices and agreed scope |
| Preparation for site | Weeks 21–24 | Required consents, neighbour procedures, contractor and key orders in place |
| Construction | Weeks 25–38 | Inspected work, services commissioned and rooms ready to use |
| Handover and initial snagging | Weeks 39–40 | Outstanding defects recorded and completion information collected |
The overlapping weeks are deliberate. Technical design and tendering can begin before every decision is complete, but doing so creates redesign risk if the approved scheme changes. Agree which work can proceed and who carries that cost before bringing it forward.
1. Agree the brief and survey
List the rooms you need, your total budget and any date you are trying to meet. Include kitchen supply, external works and temporary accommodation in the discussion. A construction-only figure cannot establish affordability.
Arrange measured information and identify questions about drains, trees, boundaries and existing structure. Agree when you will choose the preferred layout and approve the drawings. Repeated changes to the footprint or roof can require new drawings and further checks across several disciplines.
At the end of this stage, establish the planning route for the whole proposal. A design should not be labelled permitted development just to fit a preferred start date.
2. Allow for the correct application process
An ordinary householder planning application normally has an eight-week statutory decision period once valid, unless a longer period is agreed. Missing information can delay validity; a late decision does not automatically grant permission. Government determination guidance explains the decision periods and extensions of time.
A lawful development certificate also normally has an eight-week period. It is not inherently a faster planning application. Larger rear-extension prior approval has its own 42-day procedure and eligibility tests. Read the planning permission timescales guide for the different clocks and starting points.
Record the valid date, target date and any agreed extension. After a decision, review the approved drawings and conditions before authorising construction. A requirement to discharge a condition before starting belongs in the programme.
3. Prepare technical information and building control
Planning drawings do not usually contain everything needed to price and build the extension. Coordinate structural calculations, drainage, insulation, ventilation, services and construction details before comparing final tenders.
For ordinary projects, building control may be provided by the council or a registered building control approver. Government guidance gives a full-plans decision period of five weeks, or two months with consent. A building notice offers no equivalent formal approval of plans and is not suitable for every project. Choose the route for the work, rather than treating it as a way to skip design. See building regulations applications.
Agree inspection stages and notification requirements with the building control body and contractor. Work that will be concealed must be available for the necessary inspection before it is covered.
4. Start neighbour discussions early
A friendly conversation helps explain the proposal, but it does not replace a required party wall notice. Identify affected owners and the relevant work while the design is developing. Serve notices when sufficient detail is available and consider whether later revisions would require new notices.
Party structure notices normally require at least two months under section 3. Relevant new boundary-wall and excavation notices have at least one-month periods under sections 1 and 6. The applicable process depends on the work; silence is not a general agreement to proceed.
If a dispute requires surveyors and an award, allow project-specific time for that process. Do not assume it will finish within the notice period. The party wall guide explains the separate steps.
5. Select the contractor and fix the start conditions
Send the same drawings, specification and scope to each contractor. Compare exclusions, provisional sums, programme, references, insurance and how changes are priced. Establish who orders windows, doors, kitchens and other long-lead items.
Before placing non-refundable orders, confirm the design and any site dimensions the supplier requires. An early order can save time but can also become expensive if approval or structural coordination changes the opening.
Use a written contract with an agreed payment schedule, process for variations and completion arrangements. Record any retention in that agreement rather than choosing a deduction unilaterally at handover.
6. Build, inspect and prepare for handover
The main sequence is groundworks and drainage, structure and roof, weatherproofing, first-fix services, insulation and linings, second fix and finishes. Trades can overlap, but inspections, curing, drying and commissioning create dependencies. Ask for a programme showing these rather than just a finish date.
Review progress and decisions regularly. Keep a dated record of changes, costs and their effect on completion. Hidden drainage, unsuitable ground, delayed glazing and late kitchen decisions can each affect several subsequent activities.
Plan safe separation from the site if staying at home. Agree access, temporary cooking and washing facilities, security and any periods when occupation is impractical. Keep flexibility in accommodation bookings until the critical work is resolved.
At handover, record defects and agree dates to resolve them. Collect the applicable building control completion document, installation and compliance certificates, commissioning information, warranties and maintenance instructions. The documents depend on the work and certification route; a generic checklist cannot establish that the project is complete.
Start with a property check and review our current service scope and pricing. Build a programme around the actual proposal, with a named person responsible for each outstanding decision.
Frequently asked questions
Can I agree a completion date before appointing a builder?
Use it as a target for the brief, then ask prospective contractors to test it against design readiness, approvals, procurement and their availability. Agree a realistic programme and completion arrangements in the building contract. A date in an early design schedule does not by itself commit a contractor to finishing then.
What should happen if the planned start date slips?
Ask the project team to issue an updated programme showing which tasks are affected and which can still proceed. Reconfirm contractor availability, quote validity, delivery dates and accommodation arrangements before making further commitments. Record who is responsible for each outstanding decision so a revised date is supported by a clear sequence of work.