Design and Build vs Traditional: Who Carries the Gap

The 2 routes differ in one thing that actually matters: who pays when the drawings and the building disagree. Under a traditional contract that gap is yours. Under design and build it belongs to the contractor, because they own both sides of it. Neither route gives you a price that cannot change, and anyone who says otherwise is selling.

Most explanations of this choice describe the paperwork. The paperwork is not the decision. The decision is about a specific, recurring moment on every building project: the day somebody discovers that what is drawn cannot be built the way it is drawn, or cannot be built for what it was priced at.

Somebody pays for that moment. Which route you choose decides who.

The two routes, in one sentence each

What each route actually costs you

Traditional costs you the gap

The design is complete before anyone prices it, which is the appeal: you know what you are buying and contractors compete on the same information.

What it costs is that every discrepancy discovered afterwards is yours. The drawing that does not account for a real beam depth, the detail that cannot be built in the sequence drawn, the specification that reads clearly to a designer and ambiguously to a joiner — each is a variation, and on a traditional contract each is a conversation between two parties you are paying separately, with you in the middle deciding which of them is right.

Most homeowners are not equipped to adjudicate that, and it is not a criticism of them. It is a structural problem with the arrangement.

Design and build costs you design competition

One contract, one party accountable, and buildability risk sitting with the people who have to build it. That is a genuine transfer of risk and it is why the route exists.

What it costs is leverage after appointment. Their designer works to their brief. If you want to change direction, that is a variation rather than a conversation, and the person you are negotiating with is also the person building it.

If the architecture is the reason you are doing this project, that matters. If the project is a well-understood extension in a house like a thousand others, much less.

What neither route does

Neither gives you a price that cannot change. Both work to a contract sum plus variations. Design and build removes one source of variation — the gap between design and buildability — and it removes none of the others. A variation because you changed your mind is still a variation. So is one because the floor came up and the joists underneath were not what anybody expected.

Any route sold as removing variations is being sold, not explained.

The questions that settle it

1. 01Is the design the point, or is the outcome the point?. A one-off piece of architecture you will look at every day for twenty years justifies keeping design control and paying for the privilege. A well-understood rear extension usually does not, and the money is better spent on the build.
2. 02Do you already have a designer you trust?. If you do, novation keeps them and hands the buildability risk on. If you do not, appointing two parties and refereeing between them is a job you are taking on without knowing it.
3. 03Is anything unusual about consent?. A listed building, a conservation officer, an unusual planning history — these reward a designer working for you and only you, all the way through, because the negotiation is with the council rather than about buildability.
4. 04How much do you want to be in the middle?. This is the honest version of the question. Traditional routes put you between two professionals who will occasionally disagree in technical language. Some clients want that visibility. Most want a number to call.
5. 05Ask who employs the designer, in writing. On any route, at any stage. It is the single question that tells you where responsibility sits, and it is answerable in one sentence by anyone being straight with you.
6. 06Ask how variations get priced, before you sign anything. Not whether there will be any — there will. Whether they are priced and agreed before the work happens or invoiced after it. That difference matters more than the route.

Where we sit, and what that is worth knowing

We work design and build, usually novated, and we should be read accordingly.

2VP is the builder, not a marketplace: one contract, one named project lead, and no introducer fee stacked on the build cost. Our design partner is Nicholas Elias, a collaborating architect rather than an employee, and on projects where the design is genuinely the point that distinction is worth understanding before you appoint anyone.

What we would say against our own route: if you have an architect you trust and the design matters more than the programme, a traditional contract with that architect administering it is a perfectly good answer, and we are not the right call. How we run design and build sets out what we actually do, and the estimator will give you a band for the build itself.

If you want the version of this that applies to a specific job, what a London extension costs shows where the money goes on the most common project either route gets used for.

The one line to take away

Choose on who carries the gap between drawings and building — and then ask both candidates the same question: how do variations get priced, and when?