Table of Contents
- What You're Really Hiring When You Choose an Extension Company
- The Design-and-Build Model vs. the Traditional Route
- What to Look for in a House Extension Company
- Red Flags to Watch Out For
- Planning Permission: What Extension Companies Should Handle for You
- What House Extensions Cost in the UK in 2026
- How MVV Approaches the Process
- FAQs
- Final Thoughts
What You're Really Hiring When You Choose an Extension Company {#what-youre-really-hiring}
A house extension is not a product. It is a months-long process involving design decisions, planning applications, structural work, trades coordination, and budget management. The company you choose is not just delivering a finished room — they are managing your home, your money, and your daily life for the duration.
That is why asking "which extension company is best?" is less useful than asking "which company is right for my project, my budget, and my tolerance for uncertainty?" The answer depends on how the company is structured, how they communicate, and whether they take genuine ownership from the first drawing to the final snag.
This guide covers what to look for, what to avoid, and how to make a confident decision in 2026.
The Design-and-Build Model vs. the Traditional Route {#design-and-build-vs-traditional}
Most homeowners approach an extension in one of two ways.
The traditional route means hiring an architect separately, getting drawings approved, going out to tender with multiple builders, and then managing the relationship between them yourself. You get independent design input, but the coordination burden falls on you. If the builder interprets the drawings differently from the architect, you are the one caught in the middle.
The design-and-build route puts both functions under one roof. One team handles the design, the planning application, the build, and the handover. No handoffs, no gaps in accountability, no finger-pointing when something needs resolving.
For most homeowners extending a family home, that model reduces risk considerably. The team that designs your extension is the same team building it, so decisions made at drawing stage carry through to site without getting lost in translation.
What to Look for in a House Extension Company {#what-to-look-for}
Accreditation and Track Record {#accreditation-and-track-record}
Federation of Master Builders (FMB) membership is one of the clearest independent trust signals in UK residential construction. FMB members are vetted, insured, and bound by a code of practice. It does not guarantee quality, but it filters out the most unreliable operators.
Beyond accreditation, look for a portfolio of completed projects in your area — ideally with before-and-after photography and client references you can actually contact. Platforms like Houzz list vetted construction professionals with verified reviews and project images, which makes it easier to compare firms doing similar work in similar locations.
Years in business matters too. A company with a decade or more of completed projects in London has navigated planning departments, supply chain disruptions, and the full range of residential property types. That experience tends to show in how they handle problems, not just how they describe their process.
Cost Transparency Before You Commit {#cost-transparency}
One of the most common frustrations homeowners report is not getting a realistic cost figure until they are already several conversations deep with a contractor. By that point, there is social pressure to continue, and walking away feels harder than it should.
A company that gives you a credible ballpark cost upfront — before you book a consultation — is signalling something important. They are confident enough in their pricing to show it early, and they respect your time enough not to waste it.
This is not the same as a fixed quote. Early-stage estimates will always carry a range. But there is a meaningful difference between a company that says "we'll discuss costs at our first meeting" and one that lets you arrive at that meeting already knowing whether the project is financially viable.
Budget Oversight Throughout the Build {#budget-oversight}
An upfront estimate is a starting point, not a guarantee. What matters equally is how the company manages your budget once work begins.
Ask specifically: how will I know where my money is going each month? What happens if a cost changes? Who flags variations before they become expensive surprises?
The best extension companies provide structured monthly reporting that includes budget tracking alongside progress updates — not vague assurances about keeping you in the loop, but a documented record of spend versus plan, delivered on a schedule you can rely on.
A Single Team from Start to Finish {#single-team}
Ask any company you are considering: who designs the project, and who builds it? Are they the same people, employed by the same firm?
Some companies present themselves as full-service but subcontract the design to an external architect or the build to a network of independent trades. That is not inherently bad, but it does introduce the coordination risk described above. The more handoffs in the process, the more opportunities for something to be miscommunicated.
A genuinely integrated team — where the project manager who attends your design meetings is also overseeing the site — is a structural advantage. Decisions get made faster, problems get caught earlier, and you have one point of contact throughout.
Red Flags to Watch Out For {#red-flags}
Some warning signs are obvious. Others are easy to miss when you are excited about a project.
Vague pricing language. Phrases like "it depends" or "we'll know more once we start" are reasonable in context, but they should come with a clear structure for when and how costs will be confirmed. If a company cannot give you a realistic range at the outset, that is worth pressing on.
No written contract before work begins. Any reputable firm will provide a detailed contract covering scope, programme, payment schedule, and variation procedures. Starting work on a handshake is a risk not worth taking.
Pressure to decide quickly. "We have a slot next month but need a decision this week" is a sales tactic, not a scheduling reality. A company confident in their work will give you the time to decide properly.
Inconsistent reviews. A handful of five-star reviews alongside a pattern of complaints about communication or budget overruns tells you something. Read the negative reviews carefully — not just the positive ones.
No verifiable address or physical presence. A London-based contractor should have a registered company number, a verifiable address, and a track record of completed projects you can cross-reference. Online-only firms with no local footprint are worth scrutinising.
Planning Permission: What Extension Companies Should Handle for You {#planning-permission}
Most single-storey rear extensions under four metres fall within permitted development rights in England, meaning no formal planning application is required — though building regulations approval is still needed. Larger extensions, side extensions, and anything in a conservation area or on a listed building will require a full planning application.
A competent design-and-build company should be able to tell you which route applies to your project from the first conversation, and manage the application process on your behalf, including liaising with the local authority and responding to any queries.
If a company asks you to handle planning yourself, or seems uncertain about the rules for your property type and postcode, ask more questions. Planning delays are one of the most common causes of project overruns, and with the right preparation, most of them are avoidable.
What House Extensions Cost in the UK in 2026 {#cost-ranges-2026}
Costs vary significantly depending on size, specification, location, and site conditions. As a working guide for 2026:
| Extension Type | Typical Range (London) |
|---|---|
| Single-storey rear extension (small) | £45,000 – £75,000 |
| Single-storey rear extension (large) | £75,000 – £130,000 |
| Double-storey extension | £100,000 – £200,000+ |
| Side return extension | £55,000 – £95,000 |
| Wraparound extension | £120,000 – £220,000+ |
These figures cover construction but may not include VAT, architect fees, planning costs, or structural engineer fees depending on the firm. Always confirm what is and is not included in any estimate you receive.
London costs typically run 20 to 30 percent higher than the national average, driven by labour rates, material logistics, and the complexity of working in dense urban areas.
How MVV Approaches the Process {#how-mvv-approaches}
MVV is a London-based design and build contractor covering house extensions, loft conversions, property refurbishments, and development of underutilised properties. The team has more than 25 years of experience in residential construction across London, with completed projects across SW and W postcodes including Fulham and Wimbledon.
What sets MVV's approach apart starts with how a project begins. Rather than asking you to book a consultation before you have any sense of cost, MVV offers an instant online cost estimator at themvv.co.uk. You input your project details and get a ballpark figure immediately, with no obligation. It is not a quote, but it gives you a realistic starting point before you invest time in conversations.
From there, the same team handles design, planning, project management, and the build itself. There are no handoffs to a separate architect or a subcontracted build firm. The person overseeing your project from the first design meeting remains accountable through to final handover.
Monthly progress reports — including on-site photography and budget tracking — are standard deliverables, not optional extras. You know where your project stands, financially and physically, every single month. That level of ongoing budget oversight is not something most extension companies offer as a named part of their service.
If you are at the stage of comparing companies and want to understand whether a project is financially viable before committing to anything, the cost estimator is a sensible first step. Head to themvv.co.ukand use Calculate Now to get a figure for your project.
FAQs {#faqs}
What is the difference between a design-and-build company and a traditional builder?
A design-and-build company handles both the architectural design and the construction under one team. A traditional builder constructs from drawings provided by a separately hired architect. The design-and-build model reduces coordination risk and gives you a single point of accountability throughout.
How do I know if a house extension company is reputable?
Look for FMB membership, a portfolio of completed projects with client references, a verifiable company address, and clear written contracts before any work begins. Independent review platforms and professional directories like Houzz can help you cross-reference claims.
Do I need planning permission for a house extension?
It depends on the size, type, and location of the extension. Many single-storey rear extensions fall within permitted development rights in England and do not require a formal application, but building regulations approval is still required. A reputable design-and-build company should advise you on this from the outset.
What should be included in a house extension quote?
A detailed quote should cover design fees, planning costs, structural engineer fees, construction costs, materials, and VAT. Ask explicitly what is excluded so there are no surprises once work begins.
How long does a house extension take?
A single-storey extension typically takes three to five months from planning approval to completion, depending on size and complexity. A double-storey extension may take five to eight months. Design and planning stages add time before construction begins.
What is the average cost of a house extension in London in 2026?
A single-storey rear extension in London typically costs between £45,000 and £130,000 depending on size and specification. Larger or more complex projects can exceed £200,000. London costs run roughly 20 to 30 percent higher than the national average.
Why does budget oversight during the build matter?
An upfront estimate tells you what a project should cost. Budget oversight during the build tells you what it is actually costing, month by month. Without it, variations and unexpected costs can accumulate quietly until they become a serious problem.
Final Thoughts {#final-thoughts}
Choosing a house extension company comes down to one question: who is genuinely accountable for your project from the first drawing to the day you walk into the finished space?
Accreditation, reviews, and portfolios all matter. But the structure of how a company works — whether they design and build under one roof, whether they give you cost clarity early, whether they report back to you throughout the build — tells you more than any sales conversation will.
If you want to understand what your project might cost before you speak to anyone, use the instant estimator at themvv.co.uk. Calculate Now takes a few minutes and gives you a realistic starting point with no obligation.