Design and Build Contractor London

Design and build is not a type of building work. It is a way of buying it. Under a design and build arrangement one company holds both the design and the construction, so you sign one contract, get one price and have one organisation to hold responsible when something does not line up. GS Renovation Ltd works this way on domestic projects across Central and Greater London, and this page explains exactly how it works, what it costs you and where it is the wrong choice.

Design and build against the traditional route

The traditional route is architect first, then tender. You appoint a designer, they develop the scheme and obtain consents, an engineer is appointed alongside them, the drawings are completed and then the finished design is priced by two or three contractors. The traditional route gives you competitive pricing on a fixed design and an independent professional acting for you. What it does not give you is a single point of responsibility: when the drawings do not resolve on site, the designer and the contractor each have a defensible account of whose problem it is, and you are the one standing between them.

Design and build closes that gap by putting both under one contract. The trade off is that you give up some design control and some ability to compare like for like tenders, because there is no completed design to tender.

	Traditional: architect then tender	Design and build
Contracts you hold	Separate: designer, engineer, contractor	One, with the contractor
When you get a firm price	Once the design is complete, at tender	Earlier, against an agreed scope and specification
Who resolves a clash between drawing and site	You, between two consultants	The contractor, inside their own contract
Design control	High, the designer works for you throughout	Lower once the scope is fixed, changes are variations
Cost certainty before work starts	Good, on a completed design	Good on the agreed scope, but the scope must be defined properly
Best suited to	Complex, design-led or heritage projects	Well understood project types where delivery and programme matter most

Comparison of procurement routes on domestic projects. GS Renovation, 2026.

How the contract actually works

Being straightforward about our own role matters more here than anywhere else on this site. GS Renovation is a building contractor. We are not architects, structural engineers or party wall surveyors, and design and build does not change that. What it changes is who holds the appointments.

• You agree a scope, a specification and a budget with us from a survey and your brief, in writing, before anything is designed in detail
• We appoint the designer and the structural engineer, and they sit under our contract rather than yours, so the coordination between drawings and construction is ours to manage
• The design is still produced by qualified independent professionals. Structural calculations always come from a chartered engineer, never from us
• Planning and building control applications are made and tracked as programme items, with the dates in the contract programme rather than assumed
• You get one price against the agreed scope, one programme and one contact from survey through to the completion certificate
• Party wall notices are prepared for you to serve, because the Act places that duty on the building owner and it cannot be contracted away
• Snagging, certificates, warranties and the handover pack come from one place at the end

Payments, variations and the things that go wrong

The single most useful thing in any building contract is that money follows completed work. Payments should be staged against defined milestones, such as completion of substructure, structure complete and watertight, first fix complete, and practical completion. A large deposit before anything has happened protects nobody but the builder. Long lead time materials, meaning steel, joinery, stone and glazing, are the legitimate exception and should be itemised as such rather than folded into a lump sum.

Variations are where domestic projects unravel, and design and build makes the distinction sharper rather than softer. A change you instruct, a better worktop, a moved door, an extra socket run, is a variation: it gets priced and agreed in writing before it is carried out. A change needed because the design does not work is not a variation, it is ours to resolve within the contract sum, and that is precisely the risk you are paying us to carry.

The genuine exception is what nobody could see. Foundations that are shallower than expected, a rotten wall plate, asbestos in a 1960s artex ceiling, a drain that is not where the survey says it is. Those are handled as provisional sums and a contingency, both stated plainly in the quotation. A contractor who tells you there will be no unknowns on a Victorian house is telling you something about the contractor. Our budget and contingency guide explains how to set the contingency, and the renovation mistakes guide covers the rest.

What design and build does not include

This section exists because the term is oversold. Design and build is a good route for a lot of London projects and a bad one for some, and the difference is worth knowing before you sign.

• It does not remove the need for planning permission. Consent is granted by the council on the merits of the scheme, and no procurement route changes that.
• It does not remove building regulations. Notifiable work is still notified, inspected and certified by building control.
• It does not make the contractor your party wall surveyor. The Party Wall etc. Act places the duty to serve notice on you as building owner, and a surveyor is appointed separately if a neighbour dissents.
• It does not fix a price before there is a scope. Anyone quoting a firm figure before a survey and a defined specification is quoting a number, not a price.
• It does not give you an independent professional acting solely for you. If you want a contract administrator on your side of the table, appoint one, and we will work with them.
• It is not the right route where the architecture is the point. A design-led or heritage project usually deserves a designer working for you, developing the scheme properly and tendering it.
• It does not eliminate unknowns in a period property. It allocates them, through provisional sums and a stated contingency.

If you already have an architect and drawings, that is not a problem and it is not wasted. We will price and build to a completed design as a straightforward main contractor, which is the traditional route, and quite often it is the right one. Our guide to who does what explains where the architect's role ends and the engineer's begins.

What a design and build project costs in London

Design and build is priced against a scope rather than against a rate card, but the underlying benchmarks are the same ones published across this site.

Project type	Basis	Typical London figure
Full refurbishment, mid-range	Per m² of internal floor area	£1,500 to £2,200 per m²
Full refurbishment, high end or structural	Per m², including structural openings and premium finishes	£2,500 to £3,500+ per m²
Single storey extension	Per m² of new floor area, standard to mid-range finish	£2,000 to £3,700 per m²
Side return extension	Complete project on a London terrace	£60,000 to £100,000
Double storey rear extension	Complete project, from	from £75,000
Wraparound extension	Complete project on a London terrace	£80,000 to £130,000
Loft conversion	Rooflight through to mansard, by type	£30,000 to £100,000+
Basement	Complete project under a London terrace	£100,000 to £250,000+

Indicative London ranges, 2026. Trade and project costs from MyBuilder and Checkatrade; per square metre build rates from Architecture for London and Resi.

On a traditional route you would pay the designer and engineer separately, typically 10 to 15 per cent of the build cost between them plus application fees. Under design and build those appointments sit inside our price. The money does not disappear, it moves, and you should be able to see it in the quotation either way.

Who we appoint and coordinate

Holding the design does not mean doing it. It means appointing the right people and making sure what they issue actually reaches site.

• Architect or architectural designer: measured survey, scheme design, planning drawings and the construction information the trades build from
• Chartered structural engineer: beam and foundation design, underpinning sequences, temporary works and the calculations building control assesses
• Building control: notified before notifiable work starts, inspecting at the defined stages and issuing the completion certificate
• Party wall surveyor: appointed where a neighbour dissents, with the award governing how the affected work is carried out
• Certified electricians and gas engineers: Part P notifiable work and gas alterations, with certificates issued to you
• Specialist designers where the project needs them, including waterproofing design to BS 8102 on any below ground work

Under CDM 2015 the client duties on a domestic project pass to the contractor in most cases, and where more than one contractor is involved someone must be appointed principal contractor. On a design and build project that is us, and it is a fair question to put to anyone quoting for your work.