Design and build, or a designer then a builder: which suits your project?

Neither route is better for every project. Design and build puts design and building work with one company under one contract, so one party answers for the result and the stages can overlap: it tends to suit a typical London extension, loft or refurbishment. Appointing a designer first, then builders, buys independent advice and a tender on finished drawings, at the cost of a handover between the two.

MVV designs and builds, so we have a side in this. What follows is the fair version: how each route handles responsibility, price, time, control and communication, when the traditional route is the better choice, and what to ask either way.

A homeowner can run an extension, a loft conversion or a renovation in one of two ways:

• The traditional route: you appoint a designer, often an architect, to survey the house, design it and take it through planning and the technical drawings. Builders then price those drawings, and you sign a separate building contract with one of them. Your designer may stay on to inspect the work and check the builder's payments, if their appointment includes it.
• Design and build: one company surveys, designs, obtains the approvals, prices and builds, under one contract. The design is still paid for, but inside the same agreement as the build.

Only people on the Architects Registration Board's register may call themselves architects. At MVV, our own team provides the architectural design, the structural engineering, the fire engineering and the interior design, and we build it. Who you need on your side explains each role.

This is the biggest practical difference. On the traditional route, the designer answers for the design and the builder for the building work. The trouble is the problems that sit between the two, such as a detail that cannot be built as drawn, or a steel beam that clashes with a soil pipe. Each party can point, often honestly, at the other, and you hold both contracts while they settle it.

With design and build there is no one else to point at: the company that drew the detail has to make it work. Check that the contract says so plainly, and lists anything it excludes, such as drawings you supplied yourself.

Neither route removes the unknown: what is found when a floor is lifted is usually priced as it is found, so ask how each firm handles it.

Health and safety follows the same pattern. A job with more than one contractor needs a principal designer and a principal contractor under the Construction (Design and Management) Regulations 2015. On the traditional route those roles can sit with different firms; in MVV's design and build service the principal designer is included, and we build the work.

On the traditional route, the build price arrives late, but it is a competed one. You pay for the design and the technical drawings first, and builders then price the finished package on identical information. You may only learn what the design costs to build once it is finished, and a design over budget has to be redrawn and priced again.

With design and build, the price grows alongside the design: the people who will build it can price each option as it is drawn, keeping the design tied to the budget. The trade-off is that early figures are estimates, and the price is fixed only when the design is, in the contract.

On either route, a price is only as firm as the drawings behind it: anything left undecided becomes a provisional sum, and anything changed later a variation. Deciding early and building once explains why late changes cost more.

At MVV the price can come in three steps: a free estimate for a typical project like yours, the AI Estimator for your own project, trade by trade, and a quotation after a site visit. Free estimate, AI estimate or a quotation explains each one.

The design is paid for either way. With a separate designer you pay the design fees and then find a builder; with design and build, the design sits inside the same contract as the build. MVV's own fees for a single-storey extension under 100 m², indicative and ex VAT:

• Initial drawings: £800 to £1,120, covering the measured survey, design options, the planning drawings and making the application.
• Full design package: £2,000 to £3,000, adding the building regulations drawings and the building control application.

Our design costs guide sets these beside typical London prices and the fees set by law, and the design cost calculator shows which of them your own home is likely to need.

For a new house, the route can also change the VAT on the design fees: new build or refurbishment explains how HMRC treats design inside a design and build contract.

The traditional route usually runs in sequence: design, planning, technical drawings, a tender, a contract, then the start on site. Each handover takes time, and the tender adds a stage of its own: drawings issued, site visits, prices returned and compared.

Design and build lets more of it overlap. While the planning application is with the council, which has 8 weeks to decide a householder application once it is validated, the people who will build the project can complete the technical design, price it and plan the build, and there is no tender to run afterwards.

To be fair, the outside clocks are the same on both routes: the planning decision, building control, and the one or two months' notice that party wall work needs. Design and build saves the handovers and the tender, not those. What has to be in place before the builders arrive covers each clock, and planning a house extension step by step puts them in order.

The fair worry about design and build is that a design led by a builder drifts towards whatever is easiest to build. On the traditional route your designer works for you alone, and can defend the design when a builder proposes a cheaper alternative.

With design and build, you keep control by fixing things in writing before the contract: the drawings, the specification and a schedule of finishes, down to the taps and the tiles. Approve the drawings before the price is fixed, and ask for any substitution in writing. In return, designers who work beside the builders know what a detail costs and how it goes together.

At MVV, an interior design package with technical drawings and 3D visuals lets you see the rooms before a wall is moved, and the Design Studio has free design tools for thinking a project through first. Design before you build explains what the drawings decide.

Partly. On the traditional route several builders price the same drawings, so the comparison is like for like. With design and build you choose the company before the design is finished, so the competition comes earlier and on different terms: the design fees, how each company prices, the work it has finished and how it answers your questions.

You can keep a design and build price honest in three ways:

• An itemised quotation: the price broken down trade by trade, with every provisional sum and exclusion listed.
• An independent check: a quantity surveyor can review the quotation for you, for a fee.
• A budget set early: test your brief against a realistic range before the design starts, so the quotation has something to be measured against.

How to choose a building company covers the checks that apply to any builder, whichever route you take.

On the traditional route you deal with two firms, and on site your questions often run through your designer, who instructs the builder, who prices the change. That puts a professional on your side in every exchange, and adds a step to each one.

With design and build there is one conversation. At MVV a dedicated project manager stays with you at every stage, and a question on site is answered by the company that drew the house, which knows why the drawing says what it says.

On both routes the habits that matter are the same: decisions recorded, changes priced in writing before they happen, and a programme you can see.

Appointing a designer first, then builders, is likely to suit you better when:

• You already have a designer you trust, or drawings with planning permission that you want built as they are.
• The house is listed or unusually sensitive, and you want a conservation specialist leading the design and answering to you alone.
• You want an independent professional inspecting the builder's work and checking each payment on your behalf.
• The design is complete, there is time for a tender, and the keenest price on fixed drawings matters most.

Where one point of contact and overlapping stages matter more than a formal tender, design and build is usually the simpler route.

Some projects mix the two: your own designer takes the scheme to planning, then a design and build company completes the technical design and builds it, with responsibility for the design agreed in writing.

Ask a designer:

• If you call yourself an architect, are you on the ARB register?
• Does the fee include the survey, the structural design and the building regulations drawings?
• How will you check what the design costs to build while you draw it?
• Who inspects the work and checks payments during the build, and is that in the fee?

Ask a design and build company:

• Who designs the project, your own team or a firm you appoint?
• Does the contract make you responsible for the design as well as the work?
• When will the price be fixed, and which parts are provisional?
• Who acts as principal designer and principal contractor?
• If I stop after the planning drawings, can I use them?

Whichever route you lean towards, start with the numbers that do not depend on it. The design cost calculator shows which design fees, surveys and approvals your home is likely to need and adds them up, indicatively, and the free estimate gives a range for the build.

To talk it through, book a free consultation: we visit the property, talk through your brief and prepare a fully costed proposal. Our design and build service shows how we run a project from the first sketch to the handover.