Design and Build Contracts for London Extensions: How They Work and When to Use One

**Traditional procurement — the separate appointments model**:

In traditional residential construction procurement, the homeowner makes a series of separate appointments:

1. *Architect or architectural designer*: Appointed first to develop the design concept, produce planning drawings, support the planning application, and develop technical (Building Regulations) drawings and specifications 2. *Structural engineer*: Appointed (either directly or via the architect) to produce structural calculations and details for any structural elements 3. *Main contractor*: Appointed once the design and specification are substantially complete — typically by tender (pricing the drawings and specification) or negotiation 4. *Other consultants as needed*: Party wall surveyor, topographic surveyor, drainage consultant, approved inspector (Building Control)

In traditional procurement, the design intent and specification are established BEFORE the contractor is appointed. The contractor prices what is described in the drawings and spec. Variations to the scope during construction typically trigger additional cost claims.

**Design and build — the single appointment model**:

• In design and build, the homeowner makes a single primary appointment — the design-and-build contractor. The contractor:
• •Conducts the initial design feasibility
• •Develops the concept design
• •Produces planning drawings (in-house or via their architectural team)
• •Supports the planning application
• •Develops technical (Building Regulations) drawings and structural details
• •Prices the works based on their own design
• •Carries out construction

Some design-and-build contractors have in-house architectural and engineering teams. Others work through preferred architectural and engineering subcontractors who are engaged by the contractor (not directly by the homeowner).

**Key differences between the two models**:

| Aspect | Traditional procurement | Design and build | |---|---| | Single point of responsibility | No — architect and contractor each responsible for their scope | Yes — contractor responsible for design and construction | | Design quality control | Homeowner (via architect) reviews and approves design | Homeowner must manage design review themselves | | Cost certainty at appointment | Uncertain — design develops before price is fixed | Earlier price certainty if design is fixed at contract | | Who bears design risk | Architect (for design errors) / homeowner (if scope unclear) | Contractor (for their design) | | Procurement speed | Slower — separate design, tender, and appointment stages | Faster — one appointment, design develops in parallel with pre-construction | | Cost competition | Strong — same spec priced by multiple contractors | Weaker — each D&B contractor has their own design | | Flexibility for changes | Changes are variations on a fixed spec — priced by contractor | Design still developing — changes cheaper before design freezes |

**When each model makes sense for London projects**:

• *Traditional procurement is typically better when*:
• •The design brief is complex or unusual and the homeowner has strong aesthetic requirements
• •The homeowner wants independent professional oversight of the contractor's work
• •The project is large enough to justify the cost of a separate architectural appointment
• •Cost competition (tendering to multiple contractors) is commercially important
• •The homeowner is experienced with construction projects

• *Design and build typically works better when*:
• •The homeowner wants speed and simplicity — one call, one appointment, one team
• •The project is a standard project type (rear extension, loft conversion, HMO conversion) where the design brief is well-understood
• •The homeowner does not want to project-manage multiple separate consultants
• •The homeowner wants an earlier price indication before design is complete
• •The contractor has strong in-house design capability and a track record of similar projects in London

**The Employer's Requirements document — the foundation of any design-and-build contract**:

In a design-and-build contract, the homeowner is called the 'employer'. The document that defines what the employer wants — and that becomes the baseline against which the contractor's design is judged — is called the Employer's Requirements (ER).

• The Employer's Requirements can be simple or complex:
• •*Simple ER (early-stage D&B)*: A brief description of the project goals, size, accommodation, and key performance requirements (e.g., 'rear kitchen extension approximately 5m × 6m, full-height bi-fold doors to garden, Part L compliant, Building Control approved')
• •*Detailed ER (developed D&B)*: A comprehensive document including outline drawings, room-by-room schedule of accommodation, performance specifications for insulation and glazing, finishes schedule, and exclusions list

For residential projects, the ER is often relatively informal — a conversation-driven brief rather than a formal document. But wherever the ER is poorly defined, disputes arise: the homeowner expected something the contractor did not include.

*The most common design-and-build dispute in London residential projects*: The homeowner believes a finish, fitting, or detail was 'obviously' included — the contractor says it was not specified and quotes a variation. The protection against this is a sufficiently detailed ER (even for a small residential project) that specifies finishes, materials, product categories, and key design intent items.

**Who holds the design liability in a design-and-build contract?**

• In a design-and-build contract, the contractor holds the design liability. This means:
• •If a structural element fails due to an error in the structural engineer's calculations (and the SE was engaged by the contractor, not the homeowner), the homeowner has a claim against the contractor — not against the SE directly
• •If the design does not comply with Building Regulations, the contractor is responsible for remediation
• •If the planning application is refused because the design is inappropriate, this may fall within the contractor's scope to redesign at their cost (depending on contract terms)

This transfer of design liability to the contractor is one of the key risk benefits of design and build — but it only applies to the design elements within the contractor's scope. If the homeowner specifies a particular product, material, or structural approach, that specified item may sit outside the contractor's design liability.

**Standard contracts for residential design-and-build projects**:

For larger residential design-and-build projects, the JCT Design and Build Contract 2016 (JCT DB 2016) is the industry-standard contract form. For smaller residential projects, the JCT Homeowner Contract (Building Owner/Occupier edition, 2015) covers both traditional and design-and-build appointments with appropriate simple language.

• Key contract provisions to look for in any residential design-and-build contract:
• •A clear description of what is within the contractor's design scope (and what the homeowner is specifying/responsible for)
• •A design development and approval process (giving the homeowner the opportunity to review and approve designs before construction)
• •Change/variation mechanism — how changes during the design process are priced and agreed
• •Practical completion and defects liability provisions
• •Retention provisions
• •Insurance — particularly professional indemnity insurance (PI) for the design elements

**The design-and-build risk that homeowners most often underestimate**:

In traditional procurement, the architect acts as the homeowner's independent professional representative — reviewing the contractor's work for quality and compliance, certifying payments, and issuing instructions for variations and defects. In a design-and-build contract, there is no independent professional between the homeowner and the contractor. The homeowner must review designs and inspect works themselves (or appoint an independent employer's agent or project monitor to do so).

• For significant design-and-build projects, engaging an independent *employer's agent* (typically an architect or project manager) to review the contractor's designs and inspect the works on the homeowner's behalf provides important protection. The employer's agent typically costs 3–5% of the construction value and provides:
• •Independent design review at key stages
• •Site inspections at structural and critical stages
• •Certification of practical completion
• •Oversight of the snagging process

**What to look for in a London design-and-build contractor**:

Not all contractors who describe themselves as 'design and build' have genuine in-house design capability. In the London residential extension market, the quality of design-and-build services varies enormously — from highly capable integrated teams with qualified architects and structural engineers in-house, to loose arrangements where a builder subcontracts the drawings to a CAD technician with no architectural qualification.

When evaluating a design-and-build contractor in London, check:

• *Design team credentials*:
• •Is the design produced by an ARB-registered architect, a CIAT-registered architectural technician, or an unqualified draughtsperson?
• •Do they have an in-house structural engineer or do they use preferred subcontractors?
• •Can they show past examples of planning drawings and technical drawings they have produced for similar London projects?

• *Past projects in London specifically*:
• •London planning policy and local planning authority practices differ significantly from the rest of England — a contractor who mainly works outside London may not understand London-specific planning requirements (Article 4 Directions, conservation area restrictions, TPO trees, party wall complications)
• •Ask for references from past clients in the same London borough or on similar property types (Victorian terrace, 1930s semi, flat conversion)

• *Planning application track record*:
• •Has the contractor successfully obtained planning permission for similar extensions in London?
• •Have any of their applications been refused and why?
• •Do they understand permitted development limits and Article 4 Directions in the specific borough where your project is located?

• *Building Regulations and technical track record*:
• •Have their projects obtained Building Control completion certificates?
• •Do they use competent subcontractors (Gas Safe plumbers, NICEIC electricians, CE-marked structural steel fabricators)?

• *Insurance*:
• •Do they hold professional indemnity (PI) insurance for the design elements? (see `contractors-insurance-guide`)
• •What are the PI limits and when does the policy expire?
• •Standard PLI alone is not sufficient for a design-and-build contractor — PI insurance is needed to cover design errors

**The design review process — the most critical protection for a homeowner in a D&B contract**:

The contractor's design will develop through stages — concept, planning submission, technical/Building Regulations, construction information. The homeowner should build formal review and approval points at each stage:

*Stage 1 — Concept approval*: Review the proposed layout, size, orientation, and key design features. This is the cheapest stage to make changes — before planning, before technical design, before any construction.

*Stage 2 — Planning application review*: Before the planning application is submitted, review the planning drawings carefully. Changes to design after planning approval has been granted require either a non-material amendment (NMA) or a new planning application — both add cost and time.

*Stage 3 — Technical (Building Regulations) design review*: Review the technical drawings and specification before work starts. Finishes, materials, insulation specification, glazing U-values, drainage design, structural detailing — these should all be confirmed at this stage.

*Stage 4 — Mid-construction reviews*: At key stages (foundations, structural frame, roof, first-fix M&E), review progress and confirm compliance with the agreed specification. Where an employer's agent is not appointed, the homeowner should visit the site frequently during these stages.

**RCB's design-and-build approach for London projects**:

RCB Design & Build operates as a full design-and-build principal contractor for residential extensions, loft conversions, and refurbishments across Greater London. Our approach integrates design thinking, pre-construction planning, technical coordination (architecture, structural engineering, Building Control), and delivery under one managed team — reducing fragmentation and giving clients a clear, single point of accountability from planning through to completion. All second-fix materials are client-supplied unless specifically agreed otherwise in the Employer's Requirements.