Home Extension Planning Checklist: Everything You Need Before You Start

**Check 1 — Confirm you own the property and are entitled to extend**:

• •If you are a leaseholder (leasehold flat, maisonette, or leasehold house), you will need your freeholder's consent to carry out structural works or external alterations — this is separate from planning permission. The lease will specify what alterations require licence to alter. Allow 6–12 weeks and a solicitor's involvement.
• •If you have a mortgage, the lender's consent is typically required for structural alterations — check your mortgage conditions or contact your mortgage broker.
• •If the property was purchased recently, confirm the Land Registry title shows the correct curtilage (garden area) — sometimes boundary discrepancies mean the garden the buyer thought they owned is technically part of a neighbour's title.

**Check 2 — Permitted development rights (PD rights)**:

• •Check whether the property has PD rights intact: if it is a flat, a listed building, or in an Article 4 Direction area, PD rights are removed or restricted.
• •Check the planning history of the property on the LPA's online portal — look for any previous planning conditions that removed PD rights (these appear as conditions on planning permissions, e.g., 'Notwithstanding the provisions of the Town and Country Planning (General Permitted Development) Order 2015...')
• •See permitted-development-rights-guide for a full PD checklist. If PD applies, a planning application is not required — but a Certificate of Lawful Development is recommended.

**Check 3 — Check for drainage, gas, electricity and telecoms infrastructure**:

• •Public sewer location: Thames Water Asset Map (check online before design is finalised) — see drainage-guide-extensions for the build-over implications.
• •Gas main: Contact Cadent (the London gas distribution network operator) — search 'Cadent LP Gas emergency' for the pipe enquiry service. Do not excavate near a gas main without a check.
• •Electricity: UK Power Networks (UKPN) in London maintains the electricity distribution network. Their 'cable avoidance' service provides approximate underground electricity cable positions.
• •Telecoms: BT Openreach and Virgin Media both have underground infrastructure that can be affected by extension groundworks.

Practical route: Commission a ground investigation contractor to carry out a 'trial hole' and underground utilities survey before or immediately after design stage. Alternatively, commission a Dial-Before-You-Dig search (LSBUD — Lines and Safety Buried Utilities Database) — free, online, provides approximate asset positions from all registered utilities.

**Check 4 — Party wall — identify all affected neighbours**:

• •Is the proposed extension within 3m of any adjoining owner's building or structure? If yes, party wall notices will be required under the Party Wall etc. Act 1996.
• •Is the extension on the boundary line? Party wall notices required.
• •Does the extension involve excavation within 3m of an adjoining owner's structure? Possible Section 6 party wall notice required.
• •Do NOT serve party wall notices before finalising the design — notices must describe the proposed works in detail. Serving a notice and then changing the design requires a new notice.
• •See party-wall-agreement-explained for a full guide to the party wall process.

**Check 5 — Budget — gross, not just construction cost**:

The construction cost is not the total cost. Set a gross budget that includes:

| Item | Typical cost | |---|---| | Architect / architectural technician — planning drawings | £800–£2,500 | | Structural engineer | £800–£2,500 | | Planning application fee | £258 | | Building Regulations Full Plans application | £400–£900 | | Party wall surveyor (if required) | £800–£2,000 per neighbouring owner | | Thames Water Build-Over Agreement (if required) | £1,255–£2,655 | | Construction cost (contractor) | Variable | | Client-supplied materials (kitchen, tiles, floor, sanitary ware) | Variable | | Contingency (10–15% of construction cost) | Variable | | Interior design / furnishing | Variable |

**Check 6 — Appoint the right professional team**:

• For most domestic extensions:
• •An architect or architectural technician for planning drawings (and building control drawings if requested)
• •A structural engineer for foundations, steel beams, and structural calculations
• •A principal contractor (RCB Design & Build or equivalent) to coordinate delivery

• For more complex projects:
• •A planning consultant where there is policy uncertainty (Conservation Area, PD limit disputes, pre-application required)
• •A drainage engineer where there are sewer build-over or soakaway design questions
• •A party wall surveyor where party wall notices are anticipated

**Check 7 — Design decisions that affect planning**:

• Before submitting planning drawings, confirm:
• •External materials: planning officers and Conservation Area policies often require matching materials — brick matching the existing house, or slate roof matching the original. Commission a brick sample comparison before submitting.
• •Boundary distances and heights: check PD limits if relying on PD; check the LPA's local planning policies on extension heights and setbacks from boundaries.
• •Overlooking and privacy: where the extension involves new windows on an elevation facing a neighbouring property, consider the impact on neighbouring amenity — this is a common refusal reason.
• •Roof form: flat roofs may require planning justification in Conservation Areas; pitched roofs may require matching ridge height and pitch angle to the existing house.

**Check 8 — Pre-application advice**:

For any proposal with policy uncertainty (Conservation Area, large extension, side extension near boundary), obtain pre-application planning advice from the LPA before submitting. Typical fee: £200–£500. See planning-application-guide for details.

**Check 9 — Prepare for the party wall process in parallel**:

Party wall notices typically take at least 2 months to resolve (1 month notice period plus negotiation time). Serve notices as soon as the design is finalised — do not wait for planning permission before serving notices.

**Check 10 — Finalise the scope before accepting contractor quotes**:

• A quote is only as reliable as the information it is based on. Before inviting contractors to quote:
• •Architectural drawings are complete (planning drawings as minimum; Building Control drawings preferred)
• •Structural engineer has confirmed the foundation and steel beam requirements
• •Drainage strategy is confirmed (soakaway, sewer connection, or build-over)
• •Party wall position is known (notice period started or award issued)
• •Specification is clear: floor finish, ceiling finish, window specification, external materials, level of M&E integration

Contractors cannot price accurately without this information — any quote based on incomplete information contains contingency or assumptions that will later generate disputes or variations.

**Check 11 — Contract and payment schedule**:

• •Insist on a written contract. For domestic projects, the FMB (Federation of Master Builders) domestic building contract is appropriate; for larger projects (over £50,000), the JCT Minor Works Building Contract.
• •The contract should include: full scope of works by reference to drawings; payment schedule with milestone definitions; programme (start date, anticipated completion, key milestones); variation procedure; dispute resolution clause.
• •See building-contract-guide and contractor-payment-terms-guide for guidance on what a good contract includes.

**Check 12 — Insurance**:

• •Confirm the contractor has current Public Liability Insurance (minimum £2m, preferably £5m) and Employer's Liability Insurance (legally required where the contractor employs workers). Request certificates.
• •Consider taking out a Residential Building Warranty (Latent Defects Insurance) for major works over £30,000 — this provides 10-year protection against structural defects and is increasingly required by mortgage lenders when a property is later sold. See building-warranty-guide.
• •Notify your home insurer that building works are commencing — some policies require notification and may provide site works cover, or may exclude claims arising from the construction period.

**Check 13 — Building Regulations notification**:

• •Confirm whether the contractor has notified Building Control (Full Plans application or Building Notice) before works start.
• •For a Full Plans application, the inspector approves the plans before works start, which provides greater certainty. For a Building Notice, no pre-approval of plans is required but the inspector visits at each mandatory stage.
• •Keep a copy of the Building Control reference number and the name and contact details of the appointed inspector.

**Check 14 — Pre-start site setup**:

• •Confirm where contractor vehicles will park, where skips will go (skip permit required from the Local Authority if skip is placed on the highway — typically £70–£150), and where materials will be stored.
• •Confirm the contractor's approach to site security (boarding of openings, site hoarding if required by planning conditions).
• •Confirm the point of contact for day-to-day site management — who to call if there are questions or concerns.

**Check 15 — Neighbour communications**:

• Before works start, brief immediate neighbours:
• •Introduce the project (what you are doing, when it starts, how long it will take)
• •Provide the site contact name and number for any complaints or concerns
• •Confirm working hours (typically 8am–6pm Monday–Friday, 8am–1pm Saturday — check the contractor's noise management plan and any planning conditions)
• •Consider dropping a brief note to properties 2–3 doors either side and directly behind — construction noise and dust affects a wider area than the immediate neighbours