Somewhere between deciding you need more space and actually getting it, most London homeowners hit the same wall: too many companies, too many quotes, and no clear sense of who's actually going to be accountable when something goes wrong.
You call an architect. They draw something beautiful, then hand you off to find your own builder. The builder quotes based on drawings they didn't create, flags problems the architect never considered, and suddenly your six-month project has a three-month delay baked in before anyone's even touched a brick. Multiply that by a structural engineer, a party wall surveyor, and whoever's meant to file the planning application, and it's easy to see why so many extensions in London stall before they start.
This is the exact problem a design and build company exists to solve. Instead of stitching together five separate contracts and hoping everyone talks to each other, you work with one team from the first sketch through to the final coat of paint. If you're weighing up whether that model is right for your project, this guide walks through what it actually involves — the costs, the planning process, the realistic timeline, and the mistakes that trip up even well-prepared homeowners.
What a Design and Build Company Actually Does
A design and build company combines the roles that would traditionally sit with separate firms — architect, structural engineer, planning consultant, and builder — under one roof, working to one contract, with one team accountable for the outcome.
In practice, that usually covers:
- Feasibility and site analysis — working out what's actually possible on your plot before you fall in love with a design that won't get approved
- Architectural design — translating your brief into drawings that balance how the space looks with how it will actually function day to day
- Structural engineering — calculating what's needed for load-bearing walls, steel beams, or foundations, particularly relevant for loft conversions and any wall removal
- Planning permission and Building Regulations — preparing and submitting the applications, then managing the approval process with the local council
- Interior design — layout planning, cabinetry, lighting, and finishes, so the inside of the space matches the ambition of the outside
- Project management and construction — coordinating every trade on site, from groundworks to final snagging, and keeping the client updated throughout
The point of bundling all of this together isn't just convenience. It's that decisions made at the design stage directly affect what's buildable, what's affordable, and what will sail through planning versus what will get rejected. When the people drawing the plans are the same people building them, that feedback loop happens in days rather than weeks of emails between separate firms.
Design and Build vs Traditional Contractors: What's the Real Difference?
Hiring an architect and a builder separately isn't wrong, and for very small jobs it can even be simpler. But for anything involving planning permission, structural work, or a multi-trade renovation, the traditional route introduces friction at almost every stage.
Communication. With separate firms, you become the messenger. The architect's drawings need to be interpreted by a builder who wasn't in the design meetings, and any gap between "what was drawn" and "what's actually buildable" becomes your problem to resolve — usually mid-project, when it's most expensive to fix.
Accountability. If something goes wrong on a traditionally procured project, the architect blames the builder, the builder blames the drawings, and the homeowner is stuck refereeing a dispute between two companies who each have their own commercial interest in not taking the blame. With design and build, there's one contract and one team responsible for the result, so there's nowhere for that kind of finger-pointing to hide.
Cost certainty. Separate procurement often means separate quotes that don't talk to each other. A builder pricing someone else's drawings will frequently find things the architect didn't anticipate, which shows up as a variation order later. A design and build team prices its own drawings, so the number you're given at the outset is far closer to the number you actually pay.
Speed. Because design, engineering, and planning happen in parallel rather than in sequence, projects tend to move faster. Working with a single design and build team has been shown to reduce project delays by up to 30% and shave 10–15% off overall costs compared with managing separate consultants and contractors, largely because there's no lag waiting for information to pass between firms.
None of this means traditional procurement is a bad choice for every project. If you already have a trusted architect and simply need someone to build to their drawings, that route can work well. But for most homeowners tackling their first significant extension or conversion, the single point of accountability that design and build offers is worth a great deal.
How Much Does a Design and Build Project Cost in London?
Cost is the question everyone wants answered first, and the honest response is: it depends heavily on the type of work, the size of the property, and the specification you choose. That said, here's a realistic shape of what London homeowners are budgeting for in 2026.
Single-storey extensions typically range from £35,000 to £70,000+, depending on size, glazing, and finish level.
Double-storey extensions generally sit between £60,000 and £120,000, since you're effectively paying for two floors of new structure rather than one.
Loft conversions often start from around £35,000 for a simple dormer and climb from there once you add en-suites, structural steels, or a mansard roof.
Basement extensions are among the most expensive per square metre, frequently £3,000–£5,000+ per square metre, because of the excavation, waterproofing, and structural engineering involved.
New builds in London commonly run £2,500–£4,000+ per square metre for a bespoke residential build, with premium specifications pushing that figure higher still.
A few things consistently move these numbers:
- Access — narrow London streets, limited parking, and tight side returns all add time and therefore cost
- Structural complexity — removing load-bearing walls or underpinning existing foundations requires specialist engineering input
- Specification — bespoke joinery, natural stone, and high-end glazing cost significantly more than trade-standard finishes
- Compliance — inspections, Building Regulations sign-off, and party wall matters are part of doing the job properly and shouldn't be treated as optional extras
One advantage of the design and build model specifically is cost transparency. Because the same team is responsible for both the design and the build, they can price the drawings accurately from day one, rather than a builder discovering mid-project that the architect's vision costs more than anyone budgeted for.
Planning Permission: What London Homeowners Need to Know
Planning is where a lot of London projects quietly go wrong, and it's rarely because homeowners are careless. It's because the rules are genuinely complex, vary between boroughs, and change depending on things like conservation area status, prior extensions, and whether you're in a flat, a terrace, or a detached house.
A useful statistic to keep in mind: roughly 70% of London extensions require planning permission, and around 1 in 5 applications get rejected when homeowners go it alone without expert guidance. That's not because the projects were unreasonable — it's usually a case of missing documentation, underestimating a neighbour's right to object, or misjudging what falls under permitted development.
On top of planning permission, Building Regulations approval is a separate requirement that catches many people off guard. About 9 out of 10 extensions need Building Regulations sign-off before construction can even begin, covering things like structural safety, fire escape routes, insulation standards, and drainage. Planning permission tells you whether you're allowed to build; Building Regulations approval tells you whether what you're building is safe and compliant.
Structural calculations are another layer that's easy to underestimate. Over 80% of loft conversions and effectively 100% of internal wall removals require formal structural calculations from a qualified engineer before work can proceed. Skipping this step, or getting it done by someone without the right expertise, is one of the most common reasons projects stall or fail building control inspections later on.
This is where having architects, structural engineers, and planning specialists under one roof genuinely pays off. Rather than submitting an application and hoping for the best, a design and build team can shape the proposal from the start to align with what the local council is likely to approve, based on experience with similar projects in the same borough.
How Long Does a Design and Build Project Take?
Timelines vary by project type, but most London extensions and conversions follow a broadly similar structure.
- Feasibility and consultation (1–3 weeks) — site visits, initial discussions about budget and scope, and an honest assessment of what's achievable
- Design and planning (4–10 weeks) — developing drawings, refining the layout, and preparing the planning and Building Regulations submissions
- Planning determination (8–13 weeks) — the statutory period councils have to decide on a standard planning application, though this can run longer for more complex or contested proposals
- Pre-construction (2–6 weeks) — finalising specifications, ordering long-lead materials, and confirming the build programme with the site team
- Construction (12–26 weeks) — the length depends heavily on project type; a single-storey extension might take 3–4 months, while a loft conversion with structural work can run 4–6 months, and larger new builds considerably longer
Altogether, most homeowners should expect somewhere between six months and a year from first enquiry to move-in, with the planning stage often being the part that's hardest to compress. Projects that skip proper feasibility work at the start tend to lose that time back later, usually in the form of a rejected planning application or a mid-build redesign.
Common Mistakes Homeowners Make on London Building Projects
A few patterns show up again and again, regardless of project size or budget.
Choosing based on price alone. The cheapest quote on the table is rarely the cheapest project by the time it's finished. Underpriced quotes often mean corners are being planned to be cut, or the contractor is deliberately under-scoping and expects to make up the difference through variation orders once you're committed.
Skipping structural calculations to save time. It's tempting to push ahead with construction while structural drawings are still being finalised, but this almost always causes delays and rework once building control gets involved.
Underestimating planning risk. Assuming a project falls under permitted development without checking, particularly in a conservation area or on a flat, is one of the most common reasons for rejected applications and wasted months.
Not agreeing a clear payment schedule. Vague payment terms are one of the most frequent sources of dispute between homeowners and contractors. A written schedule tied to project milestones protects both sides.
Working with too many separate parties. Every additional company involved in a project is another handoff where information can get lost, timelines can slip, and accountability can blur. This is precisely the risk the design and build model is built to remove.
Ignoring party wall matters. If your project affects a shared wall or boundary with a neighbour, formal party wall agreements are often required by law, and starting work without them can lead to costly delays or legal disputes.
Not vetting the contractor properly. Out of the many contractors a design and build team might work alongside on a given project, typically only a small fraction — often just 4 or 5 out of 40-plus considered — are ones they're comfortable recommending. That level of scrutiny matters, and it's worth asking any company how they vet the trades working on your home.
Case Studies: What Design and Build Looks Like in Practice
Numbers and processes only tell part of the story. It helps to see how this plays out on real London homes.
Cobham Single-Storey Extension. A rear extension project that reworked the ground floor layout to create an open-plan kitchen and living space, balancing planning constraints typical of the area with the homeowner's brief for more natural light and better flow between indoor and outdoor space.
Streatham Wraparound Extension. A wraparound extension combining a side return and rear extension into a single cohesive space, one of the more structurally involved extension types since it typically requires steel support across two junctions rather than one.
Clapham Loft Conversion. A loft conversion that added a new bedroom and en-suite, requiring structural calculations for the new floor and roof alterations, along with Building Regulations sign-off before the space could be used.
Wimbledon Basement Build. A basement extension involving excavation beneath an existing home, one of the most technically demanding project types due to underpinning, waterproofing, and the structural engineering needed to support the property above.
Stanmore New Build. A ground-up new build development, requiring the full range of design and build services from initial planning strategy through to final construction, illustrating how the model scales beyond single-room extensions to entire homes.
Across these projects, the common thread isn't the size or style of the build — it's that design, engineering, planning, and construction were handled by one accountable team throughout, which is exactly the value proposition of design and build as an approach.
Frequently Asked Questions
What's the benefit of design and build compared to hiring an architect and builder separately? With one team handling drawings, planning, and construction, there's no gap between what's designed and what's actually built. This avoids miscommunication, keeps costs transparent, and means the project runs smoothly from start to finish without you having to referee between different firms.
Will I get a clear cost breakdown before work starts? A reputable design and build company will give you a detailed, itemised quote before construction begins, so you understand exactly what's included and what would trigger any additional cost.
How long does a typical design and build project take in London? Most projects run from six months to a year from initial enquiry to completion, though this depends on project size, planning complexity, and whether structural work is involved.
Do design and build companies handle planning permission and Building Regulations? Yes — this is one of the core advantages of the model. In-house architects and planning specialists manage the entire process, from preparing drawings to submitting applications and liaising with the local council.
How do design and build companies keep projects on budget? Because the same team creates the drawings and prices the build, there's far less risk of the kind of surprise costs that come from a builder pricing someone else's design. Clear, milestone-based payment schedules also help both parties stay aligned throughout.
Is design and build more expensive than hiring separately? Not typically. While the upfront quote may look similar, the combined efficiency of one team handling design, planning, and construction tends to reduce the variation orders and delays that inflate costs on traditionally procured projects.
Why Homeowners Choose London Design & Build
Everything above describes the design and build model in general terms. What actually matters, though, is finding a team that delivers it properly — with real accountability, honest pricing, and a portfolio that proves they can handle your specific project type.
London Design & Build is a london design and build company based at 1st Floor, Cobden House, 231 Roehampton Lane, London, SW15 4LB, with more than 100 residential projects delivered across single-storey and double-storey extensions, wraparound extensions, loft conversions, basement extensions, flat conversions, and new builds.
Their model reflects everything this guide has covered: in-house architects and structural engineers who work alongside the construction team from day one, a planning process built around what actually gets approved in London boroughs, and a project management approach that gives every client a dedicated point of contact rather than a rotating cast of subcontractors. Their own vetting process is notably strict — of the 40-plus contractors they've worked with, only 4 to 5 make the cut for ongoing recommendation, which says a lot about the standard they hold trades to.
If you're planning a loft conversion, a single-storey extension, or a full new build in London and want one accountable team from first sketch to final handover, you can browse their case studies or get a free quote to see how the numbers stack up for your own home.
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