Design and build: what to agree before work starts
Updated: Sep 22
Design and build is a delivery arrangement, not a promise of a faster or cheaper project. Before choosing it, establish who is responsible for the design, what the price includes and how changes will be managed. Those details matter more than the label.
Understand what is being offered
In design and build procurement, the main contractor takes responsibility for design and construction as defined by the contract. The extent of that design responsibility can vary. A designer introducing a builder, or several people working closely together, is not automatically the same arrangement. JCT guidance explains the importance of matching the contractor’s proposal to the client’s requirements.
Write down the outcome you expect
A useful brief describes the spaces, performance and level of finish you want. Record essential features, items you want to retain and any restrictions on working at the property. Reference images can help communicate an intention, but should not stand in for specifications or an agreed scope.
Decide which choices must be resolved before appointment and which can follow later. Where something remains open, record the allowance, assumption or process for agreeing it.
Read beyond the headline price
What design information and construction work are included?
Which surveys, approvals, consultant services and fees sit outside the price?
Which finishes, fittings or quantities are allowances rather than settled selections?
How are access, site conditions and work to the existing building addressed?
What is the procedure for assessing a proposed change before it proceeds?
Ask for exclusions and assumptions in writing. A clear comparison is more useful than treating two different scopes as competing prices for the same work.
If you are still defining a home extension or conversion, use the Extension & Conversion Cost Calculator to explore an initial budget before seeking detailed proposals. Keep its inclusions and allowances alongside the scope you ask the team to price.
Agree who checks and who decides
Clarify who develops drawings, reviews technical information, coordinates specialists and represents the client. Agree the communication route and the decisions that need your approval. Ask how the design intent will be checked against substitutions and how completion information will be assembled. These services and responsibilities should be explicit in the appointments.
Make room for changes without losing the record
Changing a room layout or material can affect work elsewhere. Before a change is accepted, ask for a written description, the cost and programme implications, and confirmation of any further design or approval work. Keep the agreed version with the project records. A conversation on site should not be the only record of an important instruction.
Choose the arrangement around your project
Design and build may suit a client seeking a coordinated delivery arrangement, but the brief, contract and team still need careful consideration. TNCO works across design, planning and delivery; our role and the responsibilities of the wider team are agreed for each appointment. Bring your priorities and any existing proposals so we can discuss an appropriate route.
Sources and further reading
General guidance, not a project-specific assessment. Requirements and guidance can change.
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