Who Actually Builds Your Extension? Design & Build vs Build Management vs Architect and Builder

By Anderson & the LCCL Construction Team | Finchley N3

Almost every company you will find when you search for an extension builder in London describes itself the same way: design and build, one team, start to finish. Some of them are. Some are design practices that introduce you to a contractor and manage the job for a fee. Some are directories that pass your details to whichever tradesman responds. The websites look alike; the contracts do not. This guide explains the four models you will meet, what each one means for who you sign with, who is liable and who carries the price risk, and the questions that tell you which one you are looking at.

We run the first model — we design, engineer and build under one contract — so read this as our view of the market. Everything below about the other models is taken from documents clients have shown us or from those companies' own published terms, and no company is named.

The point in every extension where it matters who you actually contracted with: the steel is in, the back wall is out, and the next decision costs money.

The four models, side by side

• Design & build (one company, one contract). The same firm produces the drawings, the structural engineering, the planning and building control applications and then builds the job with its own site team. You sign one contract. If something is wrong with the design or the build, there is one party responsible for both and one warranty covering both.

• Build management (two contracts). A design practice produces the drawings, then introduces a separate building contractor from its list and manages the job for a fee. You sign two contracts — one with the manager, one with the contractor. The manager's fees are usually invoiced early (typically at commencement and again once the steel is in). Defects are the contractor's responsibility; the manager offers a separate, usually shorter, structural guarantee.

• Architect plus builder (tender). You appoint an architect, take the drawings to tender, and choose a builder. Two contracts again, but you choose both parties yourself and the architect normally has no financial link to the builder. The most control, the most admin, and the price is only known once tenders come back.

• Trade directory or lead-generation site. You post the job; builders who pay the platform respond. The platform is not a party to anything. You contract directly with a tradesman, and the quality of the contract, the insurance and the warranty is whatever that tradesman offers.

Who is liable when something goes wrong

This is the question the websites never answer and the contracts always do. Under design & build the company that drew the steel is the company that fitted it, so there is nobody to point at. Under build management, if a beam is undersized the engineer's calculation is one party's work and the installation is another's, and you are the person in the middle holding two contracts. Under a directory arrangement you are on your own. It is worth reading the defects and guarantee clauses of any proposal before the price: how long is the workmanship guarantee, who gives the structural guarantee, how long is it, and is it insurance-backed or just a promise from the company.

Our own position for comparison: every project carries a 12-month workmanship guarantee and a 10-year LABC structural warranty, which is insurance-backed and survives the company, and both sit under the one contract you sign with us.

Who carries the price risk

A design & build quotation is a fixed price for the scope on the drawings. A build-management schedule of works is usually a mix of firm prices, provisional sums (an allowance that will be re-priced later — glazing is the classic one) and an exclusions list, with per-item fees for things like a structural engineer's review or a drainage build-over application. The headline is lower; the final figure is not known until the end. Our guide to reading a building quote walks through one line by line. For what a fixed-price job actually comes to, see our four worked cost examples.

The questions that tell you which model you are looking at

• “Who will I sign the building contract with?” If the answer is a company other than the one you are talking to, it is build management or a referral.

• “Who employs the people on site?” A design & build company employs or directly engages its site team. A manager introduces a contractor who employs them.

• “When are your fees paid?” Fees paid at commencement and after the steel, before the finishing trades, tell you the manager's risk ends early.

• “Which items in this quote are provisional, and what is excluded?” Ask for the exclusions list in writing before comparing prices.

• “Who gives the structural guarantee, for how long, and is it insured?” Three years from the manager is not the same as ten years from an insurer.

• “Who serves the party wall notices and who pays if a neighbour dissents?” The answer tells you how much of the admin you are actually taking on.

Anderson's advice

“Most clients underestimate the party wall timeline — it can add 3 months to a semi-detached project.” Under a single contract it is our job to serve the notices while the drawings are being finished, so the award is in place before the start date. Under two contracts it is usually listed as the client's responsibility, and the start date slips while the neighbour's surveyor is appointed. When you compare proposals, look at who owns that timeline, not just who owns the bricks.

Why we build the way we do

As LABC Building Excellence Award Winners 2024 for Best Residential Extension in London — and London Regional Winners again in 2026 for Best Conversion or Alteration to an Existing Home — both awards were given by the building control surveyors who inspected the projects, which is only possible when the people who designed the job are the people who built it. We work from Finchley N3 across London and Hertfordshire, and our design & build guide explains each stage from first sketch to handover.

What our clients say

“Super professional and on the ball at all times. All his teams are excellent and hard working. They are conscientious and want the job to be well done.”

— Ludovic, Reviews.io

Frequently asked questions

What is the difference between design & build and build management?

Design & build means one company designs and builds under one contract and is liable for both. Build management means a design practice produces the drawings, introduces a separate contractor and manages the job for a fee — you sign two contracts, and the contractor, not the manager, is liable for the building work.

Is a cheaper build-management quote really cheaper?

Often not by the end. Management schedules usually contain provisional sums (glazing, for example) that are re-priced after the structural opening is made, exclusions that come back as variations, and per-item fees. Compare the firm price for the same scope, with the exclusions list beside it.

How do I know if a company actually builds?

Ask who you will sign the building contract with and who employs the site team. If both answers are the company in front of you, it builds. If the contract is with a 'vetted' or 'principal' contractor introduced to you, it manages.

Get a figure for your own project

To book a free 30-minute consultation call about your extension, call LCCL Construction on 020 8037 7332 or get an instant estimate online — six questions, about two minutes, and you see a real range for your postcode on screen. Finchley N3 based, covering all of London and Hertfordshire.

One team, one contract, one point of contact — from first sketch to handover.