1) Define the space and how you will live through the build
Start with function: larger kitchen-diner, ground-floor WC, extra bedroom, or work-from-home space. Sketch traffic paths (fridge–sink–oven, dining to garden doors) and storage — extensions fail when layout is an afterthought.
Decide whether you will stay in the property. If so, agree a temporary kitchen and isolation from dust; if you move out, factor rent into the programme.
2) Planning route: application vs permitted development
Many rear and side extensions can proceed under permitted development within size and height limits, but flats, maisonettes, and houses in designated areas may need full planning. A Lawful Development Certificate can give peace of mind even when you believe PD applies — useful on resale.
Your designer should produce drawings suitable for either route; your builder should not start structural work until the approval path is clear.
3) Party Wall, neighbours and sequencing
If you excavate near a boundary, work on a shared wall, or cut into a party wall for steels, the Party Wall etc. Act 1996 may require notices and agreed surveyors. Build time for response periods into the programme — rushing this step creates legal risk and project delay.
4) Structure, drainage and Building Control
Foundations must suit ground conditions; East London often has made ground and services runs close to boundaries. Above ground, open-plan layouts may need steel design and fire escape / detection upgrades — your structural engineer and Building Control body should align on junction details before ordering long-lead steels.
Drainage for new WC or kitchen waste must meet Building Regulations; connecting to existing runs may affect manhole positions in the garden.
5) Budget and contingency
Industry ballparks for domestic extensions in London often start in the low thousands per square metre for build cost alone, before kitchens, glazing and VAT — exact figures depend on spec, access and groundworks. Hold a contingency for unknowns below ground and for specification upgrades once openings are formed.
6) Choose a builder with relevant delivery experience
Ask for similar completed projects, how they stage inspections, and how variations are quoted. Clear communication matters more on constrained urban sites than on open plots.
BJ Construct Ltd delivers extensions and refurbishments across London and East London. Share your postcode, property type and approximate footprint for a grounded first conversation.