How to Choose a Builder in London

Choosing the right builder is the single most important decision you will make on any construction project. In London, where the market is crowded with firms of wildly varying quality, getting this decision right can mean the difference between a project that runs smoothly and one that descends into delays, cost overruns and poor workmanship.

This guide sets out exactly what to look for, and what to avoid, when selecting a builder for your London extension, renovation or loft conversion. Whether you are planning a straightforward rear extension or a full house renovation, the principles are the same.

1. Check Accreditations and Memberships

Accreditations are the fastest way to separate serious, professionally run construction companies from the rest. They are not a guarantee of perfection, but they demonstrate that a builder has been independently assessed and meets recognised industry standards. The three most important accreditations to look for are:

• CHAS (Contractors Health and Safety Assessment Scheme). CHAS verifies that a company has robust health and safety policies and procedures. The standard CHAS accreditation confirms compliance with current legislation. CHAS Elite goes further, requiring evidence of a mature health and safety management system that exceeds minimum legal requirements. For anything beyond small jobs, CHAS or an equivalent health and safety accreditation is a good sign that a builder takes site safety seriously.
• Constructionline. A prequalification service for the construction industry, set up by the government and privately owned since 2015, which many public bodies use to check contractors. Constructionline membership means the company has been vetted for financial standing, insurance, health and safety, and organisational capability. Membership runs from Bronze through Silver and Gold to Platinum. Gold, the most widely held level, adds a fuller assessment that includes Common Assessment Standard alignment, and it is a level many local authorities and main contractors ask for when prequalifying contractors; Platinum adds an on-site audit.
• FMB (Federation of Master Builders). The FMB is the UK's largest trade association for small and medium-sized building companies. Members are independently vetted, must carry appropriate insurance, and agree to a code of practice. FMB membership also gives homeowners access to the FMB's free dispute resolution service, which is approved by the Chartered Trading Standards Institute, providing an additional layer of protection.

A builder who holds all three, particularly at the higher tiers, has invested significantly in their professional standards. It is a strong signal that the company takes quality, safety and accountability seriously.

2. Review Their Portfolio

A reputable builder will have a portfolio of completed projects that you can review. When assessing their work, consider the following:

• Relevance to your project. If you are planning a loft conversion, look for evidence of completed loft conversions, not just extensions or new builds. Every project type has its own technical challenges, and experience matters.
• Quality of finish. Look closely at the details: joinery, tiling, plastering, kitchen installations, external brickwork. The quality of the finish reveals the standards a builder works to day-to-day.
• Before and after photographs. These show the full scope of what was delivered and help you understand the builder's capability on projects similar to yours.
• Visit a completed project if possible. The best way to assess a builder's work is to see it in person. A confident builder will welcome this; if they are reluctant, that is a warning sign.

Online reviews on Google, Checkatrade and Trustpilot can also provide useful insight, but treat them as one data point among many rather than the sole basis for your decision.

3. Get Multiple Quotes and Know What to Compare

Obtaining at least three detailed quotes is standard practice. However, comparing quotes is not simply about finding the lowest number: you need to understand what each quote includes and how it is structured.

• Fixed price vs cost-plus. A fixed-price contract gives you certainty: the builder agrees to complete the defined scope of works for a set sum. A cost-plus contract means you pay the actual cost of materials and labour, plus a management fee or percentage mark-up. Fixed-price contracts are generally preferable for residential projects because they transfer risk to the builder and give you budget certainty.
• What should be included. A thorough quote should include: a detailed scope of works broken down by trade or stage; a full specification of materials; a clear payment schedule tied to milestones; the project programme with start and completion dates; confirmation of insurance; and any provisional sums or exclusions.
• Watch for exclusions. Some builders submit low headline prices by excluding items that are clearly necessary, such as structural engineer's fees, building regulations fees, scaffolding, skip hire, or drainage connections. Make sure you are comparing like for like.

If one quote is significantly cheaper than the others, be cautious. In London construction, you generally get what you pay for. A low price often means corners will be cut on materials, subcontractor quality, or project management.

4. Red Flags to Watch For

Certain behaviours should immediately raise concerns when you are evaluating a potential builder:

• No written contract. Any builder who proposes to work without a written contract is not operating professionally. A contract protects both parties and should clearly set out the scope, price, programme, payment terms, and dispute resolution process.
• Demanding a large upfront deposit. Reputable builders do not need large sums of money before work starts. A modest deposit to cover initial material orders is reasonable; demanding 30%, 40% or 50% upfront is a significant red flag.
• No evidence of insurance. Any builder with employees must by law hold employers' liability insurance of at least £5 million. Public liability insurance is not compulsory, but no reputable builder works without it; £5 million or more is common for London residential work (we carry £10 million). Ask for certificates and check they are current. Do not accept verbal assurances.
• Pressure to sign quickly. A legitimate builder will give you time to review a quote and ask questions. If you are being pressured to sign immediately, or told a price is only valid for 24 hours, walk away.
• Reluctance to provide references. A builder with a track record of satisfied clients will happily put you in touch with previous customers. Reluctance to do so suggests there is something they do not want you to discover.
• No fixed business address or company registration. Check Companies House. A legitimate building company will be registered, have a trading history, and have a verifiable address.

5. Questions to Ask Before You Commit

Before signing a contract, make sure you have clear answers to the following questions:

1. Who will manage the project day-to-day? You want to know the name and contact details of the person who will be on site managing your project. On larger projects, this should be a dedicated project manager or site manager, not the company owner splitting their attention across multiple jobs. On a home project, the Construction (Design and Management) Regulations 2015 pass your client duties as a domestic client to the contractor (or principal contractor), so ask who will take that role and how they will plan and manage site safety.
2. What is the payment schedule? Payments should be tied to completed stages of work, not arbitrary dates. Typical milestones include: foundations complete, weathertight, first fix complete, second fix complete, and practical completion. Our guide to the stages of building an extension explains what each one involves.
3. How do you handle variations and changes? Changes during construction are sometimes unavoidable. A professional builder will have a clear process for pricing and approving variations before work proceeds, with written confirmation at every stage.
4. Can I speak to previous clients? Ask for two or three references from recent projects of a similar type to yours. Contact them and ask about quality of work, adherence to programme, communication, and how problems were resolved.
5. What warranty or defects liability period do you offer? Most reputable builders offer a 12-month defects liability period after practical completion, during which they will return to rectify any snagging items at no additional cost.

6. Design-and-Build vs Separate Architect

One of the first decisions you will face is whether to hire an architect independently and then tender the project to builders, or to use a design-and-build contractor who handles both design and construction under one contract.

Separate architect: pros and cons

• Independent design oversight: the architect works for you, not the builder.
• Potentially a wider pool of design ideas and creative input.
• However, this approach requires more coordination from you, can lead to finger-pointing between architect and builder when problems arise, and often results in a longer overall programme because the design and construction phases are sequential rather than overlapping.

Design-and-build: pros and cons

• Single point of responsibility: one company is accountable for design and construction.
• Faster programme because design and construction preparation can overlap.
• Better cost control because the builder's knowledge of construction costs informs the design from day one, reducing the risk of a design that looks beautiful on paper but is unaffordable to build.
• However, you rely on the design-and-build firm for both creativity and execution, so it is essential to choose one with a strong design team.

For most residential projects in London (extensions, loft conversions, whole-house renovations), the design-and-build approach is often the more efficient and cost-effective route, provided the contractor has proven design capability, whether through its own design team or architects it works with regularly, and takes responsibility for the design under the contract.

7. Why London Homeowners Choose Alstruct

Alstruct is a design-and-build construction company specialising in residential projects across London. We hold CHAS Elite, Constructionline Gold and FMB membership, and we carry £10 million public liability insurance, which reflects our commitment to safety, quality and professional standards.

Our design team, working with your architect or ours, takes the project from initial design through planning, building regulations and construction. This one-stop approach means you deal with a single team from day one to handover, with no gaps in communication and no ambiguity about accountability. We build on JCT contracts, and every job carries our 10-year workmanship warranty.

Every project is managed by a dedicated project manager who is your single point of contact throughout. We provide a detailed construction programme before work begins, issue regular progress updates, and operate a transparent payment schedule tied to completed milestones.

We invite prospective clients to visit our completed projects and speak to previous customers. We are confident in our work, and we believe that the best way to demonstrate our standards is to let the projects, and the homeowners, speak for themselves.

Browse our completed project portfolio or learn more about our house extension, loft conversion and renovation services.

Frequently Asked Questions