London Home Extension Checklist: 20 Things to Do Before You Start

Item 1 — Confirm planning status: confirm in writing (from the LPA or a qualified planning advisor) whether your proposed extension is permitted development, requires a Prior Approval application, or requires a full planning application. Do not assume PD rights exist without checking Article 4 Directions for the specific property address. Item 2 — Obtain all necessary planning consents: if planning permission or Prior Approval is required, obtain it. Do not start any physical site works (even demolition or ground investigation) before the planning decision is issued, unless the works are permitted development under a separate PD right. Item 3 — Submit the Building Control application: submit either a Full Plans application or a Building Notice to the relevant Building Control body before the contractor starts on site. For a Full Plans application, obtain written confirmation from Building Control that the plans have been reviewed and approved (or that they have received the Building Notice and no objections have been raised) before starting. Item 4 — Serve party wall notices: if the works require party wall notices (Section 1, 3, or 6 — see the party wall guide), serve the notices at least 1-2 months before the physical works that are notifiable start. If the neighbour dissents, allow time for the party wall award process before starting the notifiable works. Item 5 — Check for asbestos (if pre-2000 construction): any building constructed before 2000 may contain asbestos-containing materials (ACMs) — artex ceilings, floor tiles, pipe insulation, roofing felts, or soffits. Before demolition or strip-out, commission a refurbishment or demolition asbestos survey (R&D survey) from a UKAS-accredited surveyor. If ACMs are present, they must be removed by a licensed asbestos contractor before the main works start. Item 6 — Check for legal charges or restrictions: some London properties have restrictive covenants on the title (recorded at HM Land Registry) that restrict the type of extension that can be built, or require consent from a third party (typically the freeholder, in the case of a leasehold property, or a management company). A conveyancing solicitor can check the title documents and confirm whether any consents are required. Item 7 — Obtain a structural engineer's design: the structural engineer's calculations and drawings must be in place before the contractor starts the structural phase of the works. Commission the structural engineer at the same time as (or before) the architect prepares the planning drawings — do not leave the structural design until after planning permission is granted.

Item 8 — Appoint the contractor on a written contract: never start a project without a written contract that defines the scope, the price, the programme, the stage payment schedule, and the variation procedure. The JCT Minor Works Building Contract is the standard form for domestic London extensions. Verbal agreements and email chains are not contracts — they leave both parties unprotected. Item 9 — Verify the contractor's insurance: before the contractor starts, obtain written confirmation of their Public Liability Insurance (minimum £5 million for a domestic extension) and their Employer's Liability Insurance (mandatory by law for any contractor employing staff on site). Ask for the insurer's name, the policy number, and the expiry date — and check that the cover is current. Item 10 — Agree the stage payment schedule: the payment schedule must be agreed before start, tied to defined milestones (groundworks complete, walls to first floor, roof structure, first-fix complete, plaster dry, second-fix, snagging), and not front-loaded. A deposit of 10-15% of the contract sum is normal; do not pay more than 25% before substantial work is visible on site. Item 11 — Agree the programme in writing: the contractor's programme (start date, key milestone dates, and the expected completion date) must be agreed in writing before the contractor starts. The programme allows the homeowner to plan their life around the construction (arranging alternative accommodation for a loft conversion, planning kitchen disruption for a ground-floor extension). Item 12 — Check contractor's Gas Safe and NICEIC registration: if the extension includes gas or electrical works, verify that the contractor employs or uses Gas Safe registered gas engineers and NICEIC- or NAPIT-registered electricians. Ask for their registration numbers and check them on the Gas Safe Register (gassaferegister.co.uk) and the NICEIC register (niceic.com/find-a-contractor). Item 13 — Budget the contingency: confirm that the homeowner has a minimum 10% contingency (15% for a Victorian property) in available funds beyond the contract sum, and that this contingency is accessible (not locked in an account with notice period) in case variation costs arise during the works. Item 14 — Confirm professional indemnity coverage for all professionals: the architect and structural engineer should each have Professional Indemnity (PI) insurance. Ask for confirmation of their PI insurance before their appointment — this protects the homeowner if a design error causes financial loss.

Item 15 — Finalise all material selections before the contractor starts: the single biggest cause of programme overrun in London home extension projects is late material selections — the contractor cannot order tiles, kitchen units, floor finishes, or bifold doors without the homeowner's final selection and approval. All selections must be finalised before the contractor starts, and orders must be placed for all long-lead items (bifold/sliding doors 4-8 weeks, kitchen units 2-6 weeks, natural stone worktops 3-5 weeks from site template, rooflights 2-4 weeks). Item 16 — Order long-lead items immediately after planning consent: bifold and sliding doors, roof lanterns, bespoke windows, and structural glazing systems all have manufacturing lead times of 4-8 weeks from order. Order these immediately after planning consent is granted — they should arrive on site at approximately the right time for installation if ordered promptly. Item 17 — Confirm the scope boundary between contractor-supplied and client-supplied materials: agree and record in the contract exactly which materials the contractor supplies (structural materials, M&E first-fix materials, plaster) and which the client supplies (kitchen, appliances, sanitary ware, tiles, floor finishes, light fittings). Ambiguity in this boundary is the most common source of claims and disputes in London domestic extension projects. Item 18 — Arrange access and protection for the works: confirm access routes to the rear of the property (through the house, through the side gate, or from the lane/alley at the rear), agree a hoarding location if the contractor is working on or near the public highway, and arrange protection for soft landscaping (grass, planted borders) that will be subject to builder traffic during the works. Item 19 — Notify your home insurer: notify your home insurer that construction works are starting. Standard home insurance policies often exclude cover during significant structural works, or require a declaration of works over a certain value. Failure to notify may invalidate the homeowner's cover during the construction period. Site risk is the contractor's responsibility (public liability insurance), but the homeowner's contents and the existing building must remain insured separately. Item 20 — Prepare a defects snagging process: agree with the contractor before works start that a formal snagging inspection will be carried out before the final payment is made, and that the final payment retention (3-5% of the contract sum) will be released only after the snagging list is signed off as complete. Including this in the written contract prevents the dispute that arises when the homeowner tries to withhold the final payment and the contractor claims the project is practically complete.

Based on the typical issues encountered in London extension projects, the following pre-start failures are most common and most costly: Not confirming planning status before starting: projects started on the assumption that the works are permitted development, only to discover mid-project that an Article 4 Direction has removed PD rights. The result is a retrospective planning application (which may require design changes) or, in the worst case, an enforcement action requiring demolition of the built work. Not obtaining a written contract before the contractor starts: the homeowner has no protection if the contractor abandons the project, disputes the scope of what was agreed, or claims that agreed-price items are extras. Not checking the contractor's insurance: the contractor has no public liability insurance and an accident or damage to the neighbour's property occurs during the works. The homeowner may be exposed to liability. Making material selections during the build: bifold doors selected and ordered 3 weeks into the build — 4-week manufacturing lead time means the structural opening sits exposed to weather for 7 weeks while the doors are manufactured. Programme delayed, builder idle, cost overrun. Not budgeting a contingency: an unexpected structural defect found during strip-out (rotted floor joist, collapsed drain) costs £8,000 to remediate. The homeowner has no contingency — the remaining project budget must be redeployed, resulting in a specification reduction or a personal finance emergency. Not serving party wall notices in time: structural works requiring a party wall award start before the award is in place. The neighbour obtains an injunction stopping the works — 6-week programme delay while the award is concluded.