How to Project Manage a Home Extension: An Owner's Guide

The decisions you make before a spade goes in the ground determine more of the project outcome than anything that happens during construction.

**Appoint the right design team**:

• For most London householder extensions, the minimum team is:
• •Architect (or architectural designer) — drawings for planning and construction
• •Structural engineer — foundation and structural steel design, Building Regulations submission support

• For more complex projects, you may also need:
• •Party wall surveyor — if works affect a party wall
• •Principal designer (CDM) — if there are two or more contractors; often the architect or a specialist CDM consultant
• •Planning consultant — for complex or contentious planning applications

• **Get proper information before pricing**:
• The quality of information you give to contractors for pricing determines the quality and reliability of the prices you get back:
• •Planning drawings for planning-only applications are not sufficient for pricing — you need construction drawings (floor plans with dimensions, sections, elevations, structural details)
• •The structural engineer's specification and drawings must be in place before you price — particularly the steel specification and foundation design
• •A detailed specification of materials, finishes, and fittings helps prevent contractors pricing to different assumptions

• **Get multiple quotes on the same information**:
• •Obtain at least three quotes from contractors, all working from the same set of drawings and specification
• •Quotes that vary by more than 20–25% suggest either a scope misunderstanding or a significant difference in programme or commercial approach — investigate before deciding
• •Never select on price alone: check references, verify insurance, confirm programme, and assess the contractor's communication and professionalism

• **Agree a contract before starting**:
• •A written contract is essential — the minimum should be a detailed scope of works, agreed price (or clear basis for a variable price), stage payment schedule, start and anticipated completion date, and provisions for variations
• •JCT Minor Works (JCT MW) or JCT Homeowner/Occupier Contract are appropriate standard-form contracts for domestic extensions
• •Do not let contractors start work without a signed contract — a handshake agreement is not sufficient protection

Once work starts, your role is to monitor progress, make timely decisions, authorise payments at the right milestones, and manage changes.

• **Set up a communication rhythm**:
• •For a project of 8–16 weeks, a weekly site visit (by you) and a weekly brief update from the contractor (verbal or written) is appropriate
• •Know who your point of contact is — site manager, contracts manager, or the principal himself
• •Keep a record of key conversations in writing — a brief email summary after a site meeting ('To confirm our conversation of today: we agreed...') creates a paper trail without being adversarial

• **Know what to look for on site visits**:
• •Is the work progressing at the rate implied by the programme? If the programme said foundations by the end of week 2 and you're in week 3 without foundations, ask for an updated programme
• •Is the site clean and well-managed? Site discipline is a reliable indicator of workmanship discipline
• •Are all the materials being used matching the specification? For example: is the brick type the one approved by planning? Is the insulation the specified product? Ask to see delivery notes if you're not sure.
• •Are building control inspections happening at the right stages? (See our guide on Building Control inspections for the required inspection points)

• **Managing variations (changes to the original scope)**:
• •Every variation — no matter how small it seems — should be agreed in writing before work starts
• •The format is simple: what the change is, what it adds or deducts from the price, how it affects the programme, and a signature from both parties
• •'Verbal variations' ('just do it and we'll sort out the price later') are the most common source of end-of-project disputes
• •Keep a variations log: list each variation, the agreed price change, the date agreed, and whether it has been incorporated into a revised payment schedule

• **Stage payments — pay on verified progress**:
• •Payment should follow progress, not a calendar
• •Before making each stage payment, confirm that the work the payment is for has actually been completed — visit the site, check the stage description matches what you see
• •The contractor will often produce a payment application — a document listing what they believe they are due. Review it against the agreed contract and progress. It is normal and appropriate to query applications that appear to be ahead of the verified progress on site

**Common mid-project issues and how to handle them**:

*Programme slippage*: The project is running behind. Ask for a revised programme in writing. Understand the cause: subcontractor delay, materials procurement, design issue, weather. Some delay is unavoidable; systematic delay without explanation is a warning sign.

*Unexpected structural findings*: The foundation digs reveal made ground, soft spots, or buried obstructions. This triggers a design review. The structural engineer must be informed; do not allow the contractor to 'sort it out' without the engineer's input and signed-off revised design.

*Neighbour complaints*: Handle promptly and directly. See our guide on managing neighbours during building work.

*Contractor cash flow problems*: Warning signs — contractor asking for payments ahead of agreed stage points, requests for 'advance on materials', complaints that suppliers aren't paid. If cash flow problems are evident, tighten payment against verified progress; do not front-load payments you may not recover.

• **Building Control inspections and completion certificate**:
• •The Building Control inspector must inspect specific stages of the build (see our guide on Building Control inspections). The final inspection triggers the completion certificate.
• •The completion certificate is the legal record that the build complies with the approved drawings and Building Regulations. Without it, you cannot prove compliance — which matters for mortgage lenders, insurance, and future sales.
• •Chase the completion certificate if it hasn't arrived within 4–6 weeks of the final inspection. Your contractor should have triggered this; if not, contact the Building Control body directly.

• **Practical completion and the snagging process**:
• •When the contractor believes the works are complete, they will request practical completion sign-off. Before you sign, carry out a thorough snagging inspection — see our dedicated snagging guide for a room-by-room checklist.
• •Agree the snagging list in writing with the contractor before signing practical completion. The list should be resolved within 2–4 weeks.
• •Only sign the practical completion certificate once you are genuinely satisfied with the state of the works — do not sign under pressure to 'be helpful'.

• **Retention and defects liability**:
• •If you have a retention in place (standard commercial practice is 5%): 50% releases at practical completion, the remaining 50% is held during the Defects Liability Period (6–12 months).
• •At the end of the DLP, carry out a final inspection and issue any outstanding defects for the contractor to remedy before releasing the retention.

• **Planning condition discharge at completion**:
• •Review your planning conditions log. Are all pre-occupation conditions discharged? If not, submit discharge applications now — before you use the new space.
• •Material samples approved? Landscaping in place? Boundary treatment installed? All must be in place and approved before occupation.

• **Celebrate — then document**:
• •Take photographs of the completed works from all angles
• •Keep all documents in one place: planning decision notice and condition discharges; Building Regulations approval and completion certificate; Gas Safe/Part P certificates; structural engineer report; architect's drawings set (as-built); warranty documents; contractor's guarantee
• •These documents will be required when you sell, remortgage, or make an insurance claim — and they will be easier to find if they are organised now rather than 10 years later