RICS recommends agreeing the work, responsibilities and dates before construction rather than leaving the project to an informal understanding. [1]
1. Survey, options and an agreed brief
Begin by establishing what the existing building and site can support. The structural engineer may need investigations where the construction, foundations or previous alterations are uncertain. Those findings can change the design, so programme them before final prices and major orders depend on an assumption. [2]
Set decision dates for the layout, roof form, kitchen arrangement and specification. Ask the programme to identify what information is needed for each next step, rather than treating all drawings as one interchangeable package.
2. The planning route
The normal statutory determination period for a householder planning application is eight weeks, measured from a valid application. This is a decision period, not a promise of approval or the duration of the whole project. An agreed extension of time or the need to revise the proposal can change the programme. [3]
The larger-home-extension prior-approval procedure instead has a 42-day decision period. It is a particular permitted development route with its own qualifying conditions, not a general six-week route for any extension. Follow the rules for the complete submission and the lawful start point. [4] [5]
From 31 October 2026, the national delegation scheme normally places householder decisions with officers, subject to statutory exceptions. It does not eliminate validation requirements or guarantee a quicker decision for an individual project. [6]
3. Technical work and pre-start requirements
Planning approval does not mean every pre-start issue is resolved. Read the decision notice: pre-commencement conditions can prevent work starting until their requirements have been met. [7]
Coordinate the engineering, building regulations submission, drainage arrangements and construction information. Thames Water says the necessary build-over agreement must be obtained before starting the affected work. [8] [9]
Party wall procedures can run alongside other preparation, but their timing must be allowed for. Existing party-structure work normally requires two months' notice; boundary-wall and qualifying excavation notices normally require one month. Consent, disputes and any award affect when the relevant work can proceed. Do not simply add every notice period together or assume they can always be shortened. [10]
4. Construction and inspection stages
Ask the contractor's programme to show foundations and drainage, the structure and roof, work connecting the addition to the house, services and finishes. Building control inspections need to be coordinated with progress, particularly before work is concealed. [11]
Confirm ordering responsibilities and supplier dates against that sequence. Record how changes to the design, discoveries on site and changes to the programme will be agreed under the contract. [1]
5. Completion and the applicable regulations
Allow for final inspections, outstanding work and the completion documents; do not define completion only as the day the main building team leaves. [8] [11]
For a programme crossing 24 March 2027, check the new energy-efficiency provisions and transitional arrangements. The published 2026 Approved Document L includes extensions to existing homes; relying only on the statement that the Future Homes Standard concerns new homes would miss that distinction. [12] [13]