The short answer
Choose design and build if you want one contract, a price early and the shortest programme, and your extension is fairly standard. Choose the traditional route (architect, then tender, then builder) if you want an independent designer working only for you, close control of the details and competing prices on the same drawings. Most homeowners in England end up in between: an architect or technologist takes the scheme through planning and building regulations, then three builders price those drawings.
If you would rather answer questions than read, the route chooser scores four routes for your project in about two minutes.
What traditional procurement means
Traditional procurement, also called design-bid-build or architect-led procurement, separates design from construction. You hold two contracts: one with the designer and one with the builder. The work runs in sequence.
- Brief and survey. You appoint an architect, architectural technologist or surveyor, who measures the house and agrees what you want.
- Design and approvals. Options, then planning drawings where planning permission or prior approval is needed, then building regulations drawings and a written specification. A structural engineer designs beams and foundations.
- Tender. The same package goes to three or four builders. Each prices exactly the same work, which is what makes the prices comparable.
- Contract. You sign with the chosen builder, usually on a JCT Minor Works or JCT Home Owner contract, with the architect named as contract administrator if they stay on.
- Build. The contract administrator inspects, answers queries, values work for each payment and certifies practical completion.
Contract administrator: the person named in the building contract to issue instructions, value work and sign certificates. On the traditional route it is your architect or surveyor. Although you pay them, they must certify fairly between you and the builder.
Advantages of traditional procurement
- An independent adviser. The designer has no stake in the builder's margin, so advice on quality, value and workmanship is yours alone.
- Competitive tendering on one design. Several builders price identical drawings, so the price is tested by the market and every quote can be checked line by line.
- Design control. You decide rooflight positions, brick, junctions and finishes before anyone prices them, and the drawings record those decisions.
- Price fixed on a full design. Because the design is complete at tender, the contract sum rests on real information rather than assumptions.
- You own the drawings. If the builder fails or you part company, another builder can pick up the same package.
Downsides of the traditional route
- It is the slowest route. Design, approvals, tender and build run one after another. Six to twelve months from first meeting to start on site is common for an extension.
- The price arrives late. You pay design fees for months before tenders tell you what the build costs. If prices come in over budget, redesign costs more fees and more time.
- Design risk sits with you. Gaps or clashes in the drawings come back as variations, and the builder is entitled to be paid for them.
- Split responsibility. When something fails, the designer may blame workmanship and the builder may blame the drawing. You may need to pursue the right party.
- Less builder input early. The builder sees the scheme only at tender, so practical savings they could have suggested arrive too late to use.
What the design and build method is
In design and build, one company takes responsibility for both the design and the construction under a single contract. For a home extension that usually means a builder with an in-house designer, or a builder who subcontracts drawings to a designer they use regularly.
- Brief and budget. You describe what you want and the company gives a budget figure, often before drawings exist.
- Design. Their designer draws the scheme, usually with the builder's preferred details and products already in it.
- Approvals. They make the planning and building regulations applications, normally inside their fee.
- Contract sum. Once the design is agreed they fix the price. A suitable JCT form is Minor Works with contractor's design (MWD 2024), or the Home Owner contract for a simple job.
- Build. They build it and solve their own design questions on site.
The main difference from design-bid-build is the order and the number of contracts: in design-bid-build the design is finished and then put out to bid; in design and build the same company designs and prices, and the stages can overlap.
Advantages and disadvantages of design and build
Single point of responsibility is the headline advantage. If the roof leaks because of a design detail, the company that drew it also built it, so there is nobody else to blame. Design can overlap with ordering and site preparation, which often saves weeks or months, and you know the price earlier.
The disadvantages are independence and control. The designer works for the builder, so when a detail could be done the expensive way or the cheap way, the choice is made inside the company that profits from the difference. You cannot easily get rival prices on the same design, because each company proposes its own. And unless you pay for your own adviser, nobody independent checks the work for you.
Read the specification, not just the price. A fixed price is only as firm as the description behind it. "Kitchen to client's choice" or "flooring allowance" can hide a provisional sum. The chapter on provisional sums shows how to find them.
Design risk, cost control and value engineering
Three ideas decide how each route behaves once the money starts moving.
Design risk
Design risk is the cost of the design being wrong or incomplete. On the traditional route you carry it, through your designer: missing details become variations you pay for, and you look to the designer's professional indemnity insurance if the error is theirs. On design and build the contractor carries it for the parts they design, normally to a standard of reasonable skill and care rather than fitness for purpose. A detailed brief from you (in JCT language, the Employer's Requirements) is still your responsibility, so if your brief is wrong, the risk comes back to you.
Cost control
Traditional procurement controls cost through competition and through the contract administrator valuing every variation against the builder's priced schedule. Design and build controls cost by fixing the price early, but changes you make afterwards are priced by the only company in the room, which is where premiums appear. On either route the habits that keep a budget intact are the same: finish decisions before signing, keep provisional sums small, ask for a price before instructing any change and hold a contingency of your own.
Value engineering
Value engineering means changing the design or specification to cut cost without losing what matters, for example a standard-sized steel instead of a bespoke one, or a flat roof instead of a lantern. Done before the price is fixed, by someone working for you, it can save thousands. Done by a contractor after you have signed a design and build contract, it can quietly mean cheaper materials. Ask for any proposed saving in writing, with what changes, what you save and what you lose.
The two routes side by side
| Question | Traditional | Design and build |
|---|---|---|
| Contracts you sign | Two: designer and builder | One: the design and build company |
| Who checks quality for you | Your architect, if retained for the build | Nobody independent unless you pay for one |
| When the price is known | After tender, on a complete design | Budget early, fixed once the design is agreed |
| Comparable quotes | Yes, everyone prices the same drawings | Hard, each firm proposes its own design |
| Programme | Longest, stages in sequence | Shorter, stages overlap |
| Design risk | Yours, via your designer | Contractor's, for their design |
| Changes mid-build | Valued by the contract administrator | Priced by the contractor, often at a premium |
| Usual JCT form | Minor Works (MW 2024) or Home Owner with consultant | Minor Works with contractor's design (MWD 2024) or Home Owner |
The routes most homeowners actually use
Drawings to building regulations, then build-only
You pay a designer for planning and building regulations drawings and hand them to three builders. The designer is not retained on site. This is the cheapest way to get comparable quotes. The catch is that nobody certifies payments or answers design queries unless you buy extra visits, so in practice you become the contract administrator.
Architect-led design and build
Your architect fixes the look, layout and the details you care about and gets planning. A design and build contractor then completes the technical design and builds for a fixed price. You keep control of what matters and pass the detail risk to the builder. On larger projects the architect is sometimes "novated" to the contractor; on homes it is more usual to hand over the drawings.
Design and build with your own adviser
You take design and build for speed and single responsibility, and pay an independent surveyor or architect to review the proposals and the contract and to inspect at key stages. A contract review and a few visits cost far less than full architect services.
What each route costs in fees
Typical ranges for domestic work in England in 2026, excluding VAT, gathered from published fee guides and practice price lists. They vary with region and scope, so use them as a sense check.
| Service | Typical range | Notes |
|---|---|---|
| Measured survey | £400 to £1,200 | Needed on every route |
| Planning drawings | £1,500 to £4,500 | Plus the planning application fee |
| Building regulations drawings and specification | £1,500 to £4,000 | Structural engineer often extra, about £600 to £2,000 |
| Full architect service with site stage | About 8% to 15% of build cost | Higher percentage on small or complex jobs |
| Independent adviser on design and build | £1,000 to £3,000 | Contract review and a few inspections |
| Design inside a design and build price | Often 3% to 8% of build cost | Shown separately or folded into prelims |
Building control charges are separate on every route; see building control and party walls.
Worked example: a £90,000 single-storey rear extension
- Traditional, full service: survey £800, architect at 11% (£9,900), engineer £1,200. About £11,900 before VAT, with inspection and certification throughout the build.
- Build-only: survey £800, planning and regulations drawings £4,200, engineer £1,200. About £6,200, with nobody independent on site.
- Design and build: perhaps £4,000 to £6,000 of design inside the contract sum, plus £1,500 if you add your own adviser.
Once an adviser is added to design and build, the fee gap narrows to a few thousand pounds. The larger difference is usually how well the price holds during the build.
How to choose: five questions
- How fixed must the price be, and how soon? A hard ceiling from a mortgage advance or savings favours early certainty from design and build. A contingency makes traditional comfortable.
- How much do the details matter to you? If you care about junctions, brick and glazing lines, keep a designer working for you.
- How unusual is the building? Listed buildings, conservation areas, basements, large openings and tight boundaries favour an independent designer.
- How soon must you start? Design and build saves time mainly by overlapping design, pricing and ordering.
- How much will you manage? Build-only suits someone who answers site questions quickly and keeps records. If that is not you, pay for the role.
Whichever route you choose, the safeguards are the same: a written contract, a defined scope, stage payments tied to work done, sensible retention and a rectification period. They are covered in JCT Minor Works and retention and stage payments.
Watch: two quantity surveyors' explainers
If you would rather hear the routes explained, these are the clearest we found. The second is American, so read the note under it.
Four procurement routes compared on cost certainty, time and design control. Management contracting and construction management are rarely used on homes, so the useful part is the comparison of traditional and design and build.
Video by Metroun Quantity Surveying. Open it on YouTube. YouTube is contacted only when you press play, through its reduced-tracking youtube-nocookie domain.
An American explainer: design-bid-build is what we call the traditional route and bids are tenders. There is nothing on JCT or UK building control, but the point about who carries design risk transfers well.
Video by Jason Schroeder. Open it on YouTube. YouTube is contacted only when you press play, through its reduced-tracking youtube-nocookie domain.
Common questions
What is the difference between design-build and design-bid-build?
Design-bid-build (the traditional route) finishes the design first, then invites builders to bid on it, giving you separate contracts with the designer and the builder. Design-build (design and build) puts design and construction with one company under one contract, so the stages can overlap and one party answers for both.
What are the advantages of traditional procurement?
An independent designer on your side, competitive prices on the same drawings, close control of the design and a price based on complete information. You also keep the drawings if you change builder.
What are the downsides of the traditional route?
It is slower, the price arrives late, you carry the risk of gaps in the design, and responsibility is split between designer and builder when something goes wrong.
Is design and build cheaper than using an architect?
Often a little cheaper in fees and quicker, but not always cheaper overall. You lose competition on one design and changes can carry a premium. Compare total cost including any adviser you add.
Can I use an architect for planning and then a design and build company?
Yes, this is architect-led design and build. Check you have a licence to use the drawings (most appointments grant one once fees are paid) and that the contractor accepts responsibility for completing the technical design.
Who is responsible if the design is wrong?
On the traditional route, the designer you appointed, under their appointment and insurance. On design and build, the contractor for the parts they designed, usually to reasonable skill and care. Get it in writing either way.
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