Design and Build or Architect and Builder in Islington?

How the two routes actually work

With design and build, one company produces the drawings, handles planning and building control, and then carries out the construction. You sign one contract and have one point of contact from the first sketch to the final snagging list.

With the traditional route, you appoint an architect who designs the scheme and secures consents. Their drawings then go out to two or three builders for competitive prices. You hold separate contracts with the architect and the builder, and usually with a structural engineer and a party wall surveyor as well. The architect can stay on to administer the building contract, or step back once work starts.

What each route tends to cost

Architects' fees for a full service on a residential project typically fall between 8% and 15% of the construction cost, with smaller and more complex jobs at the higher end. On a £150,000 rear extension and refurbishment, that could mean £12,000 to £22,000 before the engineer, surveys and party wall fees. Many homeowners trim this by paying for planning and building regulations drawings only.

Design and build firms usually charge a lower design fee, or fold it into the overall price, because they recover their margin on the build. The trade-off is that you lose the competitive tender, so it is harder to check the build price against the market. Ask for an itemised breakdown and compare it with at least one other quote.

In either case, what drives the final figure is mostly the building itself: the structural work, the specification, access for materials and skips on a tight terraced street, and how much of the existing house turns out to need attention once it is opened up.

Why Islington makes the choice matter more

A large share of the borough sits inside conservation areas, including Barnsbury, Canonbury and much of the land around Highbury Fields and Angel. Many Georgian and Victorian terraces are also listed. In these streets, permitted development rights are often restricted or removed, so work that would be routine elsewhere needs full planning permission and sometimes listed building consent.

That puts more weight on the design stage. Roof extensions, rear additions visible from the street, basement excavations and replacement windows all attract close scrutiny, and the council expects drawings that show a clear understanding of the building's character. A householder planning decision normally takes around eight weeks once the application is validated, and longer if revisions are requested.

Terraced housing also means party wall agreements with neighbours on one or both sides, which can add several weeks and a few thousand pounds in surveyors' fees. Whichever route you choose, check that someone is clearly responsible for managing this.

Which route suits which project

As a rough guide, these are the situations where each approach tends to work best.

Some homeowners take a middle path: an architect for the concept and the planning application, then a design and build contractor for the technical drawings and construction. This can work well where the planning case is the hard part and the build itself is conventional.

• Design and build suits straightforward rear or side return extensions, loft conversions, and internal refurbishments where the layout is fairly settled.
• Design and build suits you if you want one firm accountable for both the drawings and the outcome, with a fixed price agreed early.
• A separate architect suits listed buildings, sensitive conservation area sites and basement projects where the planning argument needs careful handling.
• A separate architect suits you if the design is unusual or highly bespoke, or if you want an independent professional checking the builder's work and certifying payments.

Questions to ask before you commit

Whoever you speak to, the same few questions will tell you a great deal about how the project is likely to run.

• Who produces the drawings, and are they a qualified architect or architectural technologist?
• Have you worked on similar properties in Islington conservation areas, and can I visit one?
• What exactly is included in the price, and what is listed as a provisional sum?
• Who deals with planning, building control, the structural engineer and party wall notices?
• What form of contract will we sign, and how are variations priced and agreed?
• Do I own the drawings if we part ways after planning?