My Trusted Builder · London Design & Build · Updated June 2026 · Alexandr Vreme, Director

Design & Build vs Traditional Contract: Which Protects Your Budget on a London Renovation?

You have planning consent, a scheme you believe in, and a contractor shortlist. Before you issue a single tender document, one decision will shape whether your project lands on budget, on time, and without a dispute: which procurement route are you using? The choice between a JCT Design & Build Contract and a traditional JCT Standard Building Contract is not just administrative — it determines who carries risk, who owns the design, and who answers when things go wrong.

Why procurement route matters more than most clients realise

In London, residential renovations in the £100,000–£500,000+ range routinely overrun — not because contractors are dishonest, but because the contractual structure creates conditions for cost escalation. A client who has engaged an architect on a traditional basis, then separately tendered a builder at Stage 4 or 5, has split the design liability from the construction liability. When a detail does not work on site, both parties point at the other. The client absorbs the delay, the variation costs, and the legal exposure.

Understanding the two mainstream routes — and a hybrid used increasingly for residential work — is essential groundwork before any contract is signed. This article explains both in plain terms, with specific reference to how they operate on London projects.

The traditional JCT route: design by the client's team, build by the contractor

Under the JCT Standard Building Contract (often called the "traditional" or "lump sum" route), the client's design team — typically an architect, structural engineer, and M&E consultant — produces a complete set of information. The contractor then prices and builds exactly what the drawings specify. Design liability sits with the design team. Construction liability sits with the contractor.

In theory, this gives clients maximum design control and a clearly competitive tender. In practice, on a London renovation, it carries three recurring problems:

• Incomplete information at tender. Residential projects — particularly retrofits of Victorian or Edwardian stock — rarely achieve 100% design completeness before works begin. Every gap in the specification becomes a potential variation, priced by the contractor at the moment of greatest leverage.
• Split liability. If a structural detail specified by the engineer proves unbuildable, the contractor will issue a variation notice. Whether the cost is recoverable from the design team depends on whether professional negligence can be demonstrated — a lengthy and expensive argument.
• Slower mobilisation. The full design must be substantially complete before the contractor can be appointed. On a fast-moving London project, this adds months to the programme.

Traditional procurement remains appropriate — and in some cases preferable — where the client's architect has deep familiarity with the site, the project is highly bespoke in design terms, and the client values separating creative control from construction delivery. An experienced contractor operating under a traditional JCT can work effectively with an architect's team from Stage 4 onwards, managing the build without carrying design risk.

JCT Design & Build: single-point responsibility explained

The JCT Design & Build Contract 2016 (D&B) transfers both design completion and construction to a single contractor. The client (referred to as the "Employer") sets out what they want in the Employer's Requirements — a document that defines the project's scope, quality standards, performance criteria, and any design elements the client wishes to specify or retain. The contractor responds with Contractor's Proposals, accepting responsibility for developing the design to a standard that meets those requirements.

Two features define the D&B contract in practice:

Single-point responsibility. If the roof leaks because a detail was specified incorrectly, or because it was built incorrectly, the client has one party to pursue. There is no finger-pointing between architect and builder. The contractor carries design liability for anything within the scope of the Contractor's Proposals, alongside the construction liability they hold in any route.

Fixed price at contract execution. Because the contractor has priced not just the build but the design risk, the contract sum has a higher degree of finality. Variations can still arise — particularly if the client changes the Employer's Requirements — but the structural cause of cost escalation (incomplete design at tender) is substantially removed.

The hybrid: contractor's design portion under a traditional contract

A third approach sits between the two routes: the Contractor's Design Portion (CDP) within a JCT Standard Building Contract. Here, the majority of design remains with the client's architect, but specific elements — often specialist packages such as structural steelwork, glazing systems, or M&E coordination — are designated as the contractor's design responsibility.

CDP provisions allow risk to be allocated where it logically belongs. A steel fabricator who designs their own connection details is better placed to carry liability for those details than an architect who specified a generic beam schedule. For London renovations involving bespoke structural interventions — rear extensions with large-span steels, basement underpinning, complex roof alterations — a traditional JCT with carefully drafted CDP schedules can achieve the clarity of D&B for the highest-risk elements whilst preserving architect-led design control for the rest of the project.

The risk is in poor drafting. CDP schedules that are vague about the boundary between employer-design and contractor-design simply recreate the liability ambiguity the CDP was meant to resolve. An in-house QS with contract management experience can specify the boundary precisely.

Cost certainty: what the numbers look like in London (2026)

Indicative figures for a typical London whole-house renovation or substantial rear extension in Zones 1–4:

• Traditional JCT, Stage 4–5 tender: Contractor's prelim and overhead allowances typically run 12–18% of net build cost on projects £150k–£400k. Variation instructions on an average London retrofit project routinely add 8–15% to the original contract sum where design information was incomplete at tender.
• JCT Design & Build: The contractor's D&B premium (to cover design risk) is typically 3–6% above an equivalent fully-designed traditional price. However, the removal of post-contract variation risk and the reduction in professional fee duplication (no separate post-contract architect administering the traditional contract) frequently makes the final outturn cost lower.
• Value engineering window: On a D&B contract, value engineering decisions — materials, structural systems, specification equivalents — can be resolved before the contract sum is fixed. On a traditional contract tendered at Stage 4+, value engineering after award requires a formal variation instruction, which carries an administrative cost and a contractor margin on the saving.

For a £350,000 project, the difference between a well-managed D&B outturn and a traditional contract that accumulates modest variation instructions throughout can exceed £30,000–£50,000. That is not a marginal consideration.

Design control and quality: a common concern addressed

The objection most often raised by clients and architects against Design & Build is loss of design control. It is a legitimate concern, and it is not unfounded — D&B executed without rigorous Employer's Requirements can result in value-engineered specification substitutions that compromise the architect's original intent.

The remedy is not to avoid D&B but to write Employer's Requirements that are specific enough to prevent substitution on the things that matter. Materials, finishes, performance standards, and aesthetic parameters can all be locked in. An experienced D&B contractor who values architect relationships — and depends on them for repeat referrals — will not seek to undermine the specification for marginal gain.

What D&B does genuinely change is who coordinates and resolves the technical interfaces: structural details, service routes, junction details between trades. Under a traditional contract, that coordination falls to the architect and their sub-consultants, often after the fact. Under D&B, it sits with the contractor's site management team and the contractor's design consultants, resolved before work begins rather than discovered on site.

Programme: where D&B typically accelerates delivery

On a London renovation, programme delay costs money in two ways: extended preliminaries (site management, scaffolding, hoarding, welfare facilities) and delayed occupation or sale. Extended prelims on a £250,000–£350,000 project run approximately £800–£1,400 per week, depending on project complexity and central London location premiums.

Under a traditional route, the contractor cannot be appointed until the design is substantially complete. Under D&B, the contractor can be brought in at an earlier stage, contributing to design development and beginning enabling works, groundworks, or procurement lead-time items (structural steel, windows, specialist joinery) whilst design is still being finalised.

For London projects with long-lead items — bespoke steel-framed rear extensions, replacement sash windows in conservation areas requiring Listed Building Consent approval, or custom kitchens — early contractor involvement under a D&B framework can compress the overall programme by 4–10 weeks. At the costs above, that is material.

Comparison at a glance

How My Trusted Builder structures this for London projects

My Trusted Builder operates as a design-and-build contractor across Zones 1–4 London, taking projects from planning through to handover under a single contract and a single point of contact. Our in-house team includes quantity surveying, estimating, tender management, value engineering, contract administration, and project management — meaning the commercial rigour is applied internally, not outsourced to a third-party QS who has never set foot on the site.

For projects where an architect is already engaged — whether at RIBA Stage 1, 2, or 3 — we work alongside them. Our preferred approach is to engage early: contribute buildability input during design development, flag programme or cost implications of specification choices before they are locked in, and prepare Employer's Requirements that protect the architect's design intent whilst creating the contractual framework for a fixed-price delivery.

We do not require architects to step back from projects. We require them to work with us towards a shared outcome: a London renovation that is delivered to the standard the client was promised, within the budget the client agreed, without the variation instruction cycle that undermines trust in the industry.

Our structural guarantees run for up to 20 years. Our contract model — fixed price, one contract, one contact from planning consent to handover — reflects where we place risk: with ourselves, not with the client.

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