Managing a London Extension Project 2025: Client's Guide to Running a Build

**Why every London extension project needs a written contract**:

The most common source of dispute in London residential building projects is ambiguity — about what was included in the price; what the programme commitment was; who was responsible for a specific item; what the process was for cost changes; and who bears responsibility for a defect. A written contract eliminates most of this ambiguity before the first spade goes in the ground. Many London homeowners proceed on the basis of an email exchange, a quote on headed paper, and a handshake — and find, when disputes arise mid-project, that they have no clear contractual basis to resolve them.

**Standard contract forms for residential building work in London**:

• *1. JCT Homeowner Contract (formerly Minor Works Contract)*:
• The Joint Contracts Tribunal (JCT) publishes a 'Building Contract for a Home Owner/Occupier' — a plain-English contract designed for exactly this use case: a homeowner employing a contractor for domestic building work. It covers:
• •Scope of works (by reference to drawings and specification)
• •Contract sum and payment schedule
• •Start date and completion date
• •What happens if the contractor is late (no penalty clause by default — homeowner must add one)
• •Insurance responsibilities (contractor must maintain PLI; ELI; contractor's all-risks during the works)
• •Dispute resolution (adjudication)

Cost: available from the JCT website for approximately £35–£50 for the published form. Widely understood by contractors and solicitors. Recommended for projects above approximately £10,000.

*2. Federation of Master Builders (FMB) Contract*: The FMB publishes a consumer contract for its members — FMB members are required to use the FMB contract or an equivalent written contract for works over a minimum value. The FMB contract is similar in structure to the JCT Homeowner Contract and includes mediation as the primary dispute resolution mechanism.

• *3. Bespoke or contractor-provided contract*:
• Many larger London contractors provide their own contract terms. If the contractor provides the contract, the homeowner should review it carefully and ensure it covers:
• •Defined scope (referenced drawings and specification — not just 'as quoted')
• •Fixed price vs. cost-plus pricing (is the price fixed or subject to variation?)
• •Payment schedule (agreed milestone-based payments — not a continuous weekly draw)
• •Programme and completion date with a commitment from the contractor
• •Variation order process (written sign-off required before additional costs are incurred)
• •Defects period (the period after practical completion during which the contractor is obligated to return and remedy any notified defects — typically 6 months to 1 year)
• •Insurance requirements
• •CDM 2015 duties allocation (the contract should identify who is the Principal Contractor and who is the Principal Designer — or confirm that the contractor is taking both roles)

**The payment schedule — the most important financial protection for the homeowner**:

• Payment disputes are the most common cause of project failure in London residential construction. The payment schedule should be:
• •**Milestone-based, not time-based**: payments tied to the completion of defined stages of work (after groundworks and slab; after wall plate and roof structure; after first-fix M&E; after plastering; after second-fix) rather than weekly or monthly payments on a time basis
• •**Proportionate to the work completed**: each milestone payment should not significantly exceed the value of work completed at that stage — the homeowner should never be significantly ahead of the contractor in value terms
• •**The final payment retained until practical completion and snagging sign-off**: typically 5% of the contract sum is held as a retention until practical completion; and 2.5% retained for the defects period. This provides a financial incentive for the contractor to complete the final 10% of the work and to return to address snags
• •**No deposit exceeding 10–15%** of the contract sum: a large upfront payment (above 15–20% of the total) reduces the contractor's financial incentive to begin work promptly and creates risk for the homeowner if the contractor becomes insolvent. An initial 10% deposit to cover mobilisation costs (materials; scaffolding; plant hire) is standard. Requests for 25–50% upfront are a significant warning sign in London residential construction

**CDM 2015 — the legal duties that apply to domestic construction projects**:

The Construction (Design and Management) Regulations 2015 (CDM 2015) apply to all domestic construction projects that last more than 30 working days (with more than 20 workers simultaneously) OR more than 500 person-days. For most London rear extensions (which typically involve 3–8 workers over a 10–20 week build programme), CDM 2015 applies.

• For domestic projects (where the client is not a commercial organisation), some CDM duties are modified — specifically, the 'domestic client' can transfer their CDM duties (which would otherwise include appointing a Principal Designer and Principal Contractor and notifying the Health and Safety Executive) to the contractor. In practice:
• •Where a principal contractor takes on the full design-and-build responsibility, they take on both the Principal Contractor and (by written agreement) the Principal Designer roles under CDM 2015
• •The domestic client (homeowner) should confirm in writing in the contract which party is the Principal Contractor and which is (or who is taking on the duties of) the Principal Designer
• •HSE notification (the F10 notification) is required for projects of sufficient size — the Principal Contractor is responsible for submitting this

• Practically, the main CDM 2015 requirements that affect a London extension project are:
• •A Construction Phase Plan (CPP) must be prepared by the Principal Contractor before construction begins — this documents the site safety rules, emergency procedures, first aid arrangements, and risk assessments for the project
• •The Principal Contractor is responsible for site safety for all workers on site — including subcontractors
• •The Principal Designer is responsible for coordinating design health and safety information (pre-construction information pack) and for ensuring that foreseeable health and safety risks arising from the design are addressed

**Site meetings — the most important management tool for a London extension project**:

Regular, structured site meetings between the client (or the client's architect or project manager) and the principal contractor's site manager are the most effective way to keep a project on programme, identify problems early, and maintain good communication. Meeting frequency recommendations:

• •**Weekly during active phases** (groundworks; structure; first-fix M&E; plastering): one structured meeting per week with the site manager, lasting 30–45 minutes, produces a written action list, programme update, and agreement on any outstanding client decisions
• •**Bi-weekly during slower phases** (external works; decoration; second-fix): the pace of work slows; meetings can be bi-weekly without losing control
• •**Pre-start meeting (before day 1)**: the most important single meeting. The pre-start meeting confirms: the contract is signed; all required statutory appointments have been made (Building Control application submitted; Party Wall Notices served or Awards received; CDM parties confirmed); the access arrangements for the site; the neighbour communication plan; the programme and key milestones; the payment schedule; the contact details for all key parties; the site rules. A well-run pre-start meeting prevents the most common first-week problems

*What a good site meeting should cover*:

1. **Progress against programme**: what was planned for last week; what was actually completed; what is planned for next week. Red flag: if progress is consistently behind programme without an agreed reason, investigate immediately — consistent slippage in the first 4 weeks usually predicts significant delay at practical completion

2. **Pending client decisions**: any decision that is required from the client within the next 2 weeks (tile selection; sanitaryware model; kitchen dimensions for first-fix; external lighting positions). Client decisions that arrive late regularly cause programme delays — the contractor often cannot proceed with the next stage until the client has confirmed a choice that affects that stage

3. **Variation orders (VOs)**: any works outside the original scope that have been instructed or discovered since the last meeting. A variation order should be: identified and described in writing; costed (with the contractor's written estimate before the work proceeds); approved by the client in writing; and recorded with a running total of variations approved to date

4. **Building Control inspection milestones**: when the next Building Control inspection is required; whether it has been booked; what needs to be ready for it

5. **Material and procurement lead times**: any materials that need to be ordered more than 2–4 weeks in advance (structural steels; bespoke windows; bifold doors; specific tiles; kitchen cabinets). Materials that arrive late are among the most common causes of programme delays in London extensions

6. **Issues, risks, and surprises**: any unexpected conditions (unexpected services found in excavation; condition of existing masonry worse than expected; building control query on a structural detail). Early identification of problems — and agreement on how they will be resolved — prevents small surprises from becoming large disputes

**Variation orders — the financial risk that most clients underestimate**:

Variation orders (VOs) — instructions to the contractor to carry out work outside the original agreed scope — are the primary source of budget overrun in London extension projects. They arise from:

• •**Client-initiated changes**: the client changes their mind about a specification (different tile; different sanitaryware; additional recessed lighting); requests additional items not in the original scope (a utility room cupboard; a TV aerial point; an additional socket); or decides after plastering that a door position should move
• •**Contractor-discovered unforeseen conditions**: works that could not have been reasonably priced from the original information (asbestos in the existing structure; rotten floor joists discovered when the floor is opened; hidden services in an unexpected position; existing drains in worse condition than assumed)
• •**Design developments**: the architect or structural engineer changes a detail after the main contract is signed; building control requires an additional element; the Party Wall Award requires additional works

*The golden rule for variation orders: never allow additional work to proceed without a written variation order signed by both parties*. Verbal instructions to a contractor to 'just do that as well' — without a written cost agreed — create two problems: (a) you do not know how much the additional work costs until the invoice arrives; (b) the contractor may genuinely have priced the additional work at a higher rate than you expected because they were not asked to compete on it. Every VO is a small untendered mini-contract — and contractors are human: they will price untendered VOs at their most comfortable margin.

Practical VO management: 1. When a possible VO is identified, the client tells the contractor in writing (email is fine) that additional work may be required and asks for a written estimate before any work proceeds 2. The contractor provides a written estimate for the additional work 3. The client approves the estimate in writing before the work proceeds 4. The approved VO is added to a running total — the client should maintain their own running total of all approved VOs alongside the original contract sum to track the anticipated final cost

**Programme management — what to do when a project is running late**:

• Construction projects in London are frequently delayed. Common causes specific to London:
• •**Access and delivery constraints**: narrow streets; parking restrictions; scheduled deliveries; scaffold permits; skip hire permits
• •**Party wall delays**: waiting for an Award that is taking longer than anticipated; an obstructive neighbour holding up progress
• •**Building Control delays**: inspector availability; queries on structural details requiring engineering responses
• •**Material delays**: long-lead items (structural steel; bespoke glazing; custom kitchen cabinets) arriving late
• •**Weather**: prolonged rain affecting external and groundworks; extreme cold affecting concrete pouring and plastering

When a project falls behind programme, the homeowner's appropriate response is: 1. Identify the specific cause of delay in writing at the next site meeting 2. Determine whether the delay is within the contractor's control (poor planning; insufficient labour; material not ordered on time) or outside it (weather; unforeseen conditions; client decisions pending) 3. Agree a revised milestone programme showing how the lost time will be recovered or accommodated 4. If the delay is contractor-caused: record it formally and note the contract completion date — if delay damages (liquidated damages) are specified in the contract, the mechanism for claiming them begins from the contractual completion date

For most London residential projects, the most effective approach to delay is not legal threat but practical collaboration: understanding the constraint and working with the contractor to find a solution. Contractors who feel unfairly blamed or legally threatened typically become less cooperative — which makes delays worse. Contractors who feel their client is engaged, fair, and solution-oriented typically make extra effort.

**What is practical completion?**:

Practical completion (PC) is the point at which the works are sufficiently complete for the client to take occupation and use the space for its intended purpose — even if minor items remain to be completed. Practical completion is not the same as 100% completion: it is the point at which the space is habitable or useable, with only minor outstanding items (snagging items) remaining.

• In a JCT contract, practical completion is formally certified by the contract administrator (typically the architect, if one is engaged) or agreed between the parties. At practical completion:
• •The client takes possession of the works
• •The defects liability period begins (typically 6–12 months)
• •The contractor's obligation to maintain site insurance passes back to the client
• •The retention (typically 2.5–5% of the contract sum) is reduced — half the retention is typically released at practical completion; the other half is released at the end of the defects period
• •Risk of damage to the works transfers to the client (the contractor is no longer responsible for insuring the finished work against fire, flood, or accidental damage — the client's buildings insurance must be in place from PC)

**The snagging process — how to do it properly**:

Snagging is the process of identifying, documenting, and agreeing the minor defects and incomplete items that remain at practical completion. A well-managed snagging process avoids the post-completion disputes that arise when a contractor believes they have finished but the client identifies problems months later.

• *When to snag*:
• •**Before the final 5% payment** is made — not after. Never release the final payment or final retention before a snagging inspection has been completed and agreed
• •Do the snagging inspection at a time when you can view the works in good daylight (not at dusk; not with only electric lighting)
• •Allow yourself adequate time — a thorough snagging inspection for a full rear extension or loft conversion takes 1.5–3 hours

*How to conduct a snagging inspection*:

1. **Start externally** — check: pointing quality at the new brickwork; flashing and roof junctions; external render finish (no cracking; no hollowness — tap test with a key); drainage outlets properly set; external lighting positions; any scaffold damage to existing structure; gutter and downpipe connections

2. **Ground floor internal — systematic room-by-room**: - Walls: any cracks in new plasterwork; any hollowness (tap test); any uneven areas; any unfinished plaster at junctions - Ceiling: any cracks; any movement joints; any paint defects - Floor: any unevenness; any movement between floor boards; any grout cracking in tiling - Windows and doors: correct operation; seals in place; draft-free; trickle vents installed; correct ironmongery - Electrical: all sockets and switches tested (use a socket tester); all lighting working; all circuits labelled in the consumer unit; Part P certificate issued - Plumbing: all taps operating; no drips at any connection; hot water temperature at taps; tiling to sanitary areas complete; silicone seals at all bath/shower junctions - Joinery: skirting and architrave correctly fixed; mitres cut well; all painted surfaces to specified finish; no gaps at junctions

3. **Record all items photographically** — photograph every defect or incomplete item. Written list plus photograph is the definitive snag record

4. **Agree the snag list with the contractor** — present the snag list to the contractor at the end of the inspection; agree which items are genuine snagging (to be rectified) and which items are normal characteristics of the material (hairline shrinkage cracks in new plasterwork are normal — they are not snagging; significant cracks or hollow areas are). The distinction should be agreed at the inspection, not disputed in correspondence months later

5. **Programme the snagging works** — agree a specific date by which the snagging items will be complete. Snagging often drags on indefinitely without a deadline; a specific date (agreed in writing, confirmed in the site meeting minute) creates accountability

**The defects period and the final retention release**:

• After practical completion, the defects period (typically 6–12 months as agreed in the contract) gives the contractor the opportunity to return and make good any defects that become apparent in the first season of occupation. Common defects that appear after PC include:
• •Plaster shrinkage cracks that appear as the building dries out (particularly around window and door heads; at wall-ceiling junctions; above structural lintels)
• •Tile grout cracking at movement joints or at structural junctions
• •Settlement of new doors and windows (slight movement in the structure as it settles and dries out; may require door adjustment)
• •External pointing cracks at weaker points

At the end of the defects period, the client (or architect) issues a 'making good defects' certificate confirming that notified defects have been made good — after which the final retention is released to the contractor.

A contractor who is good at practical completion but poor at snagging follow-up is a common London experience. The financial mechanism (retention withheld until defects are resolved) is the most effective lever — ensure the retention sum is meaningful (2.5–5% of the contract sum) and is not released until defects are made good to the client's satisfaction.