A house extension can transform how you live, but the most important decisions happen before anyone starts digging. What problem are you solving? What does the budget need to include? Can the design be approved and built on your site? And how will the work affect daily life?
Answering these questions early helps you avoid two common mistakes: designing more than you can comfortably afford, or spending heavily without fixing the weaknesses of the existing house.
This is the practical checklist we would use at the start of an extension project in London or Surrey.
1. What problem must the extension solve?
“We need more space” is a starting point, not a brief. Be precise about what is not working now.
Is the kitchen too narrow for more than one person? Is there nowhere to eat together? Do you need another bedroom, a quiet workspace, better storage or a stronger connection to the garden? Perhaps the house has enough floor area, but poor circulation and disconnected rooms make it feel smaller than it is.
Write down the three outcomes that matter most. For example:
- a kitchen where two people can cook comfortably;
- a dining table for eight that does not block the route to the garden;
- a utility area that hides laundry and household storage.
This makes it easier to compare designs and control the budget. A well-planned internal reconfiguration with a modest extension can sometimes solve the problem better than simply building the largest footprint available.
Also think ahead. If you expect to stay for ten years, consider how the space may need to adapt to children growing up, more home working or reduced mobility. The aim is not to predict everything, but to avoid a layout that answers only today’s problem.
2. What can I realistically afford — from design to handover?
Your construction budget is not the same as your total project budget. The full figure may also need to cover professional design, structural engineering, planning or certificate fees, Building Control, Party Wall surveyors, surveys, kitchen and bathroom fittings, flooring, lighting, landscaping and VAT.
There may be site-specific costs too. Restricted access through a terraced house, drainage diversions, deeper foundations, asbestos removal or a major structural opening can all change the price.
For context, Dali Pro Services currently guides clients towards approximately £2,500–£4,000 per square metre for a finished London extension, including kitchen and finishes. That is a broad starting range rather than a quotation. Specification, structure, access and site conditions matter more than a simple cost-per-metre calculation.
Keep a contingency for information that cannot be confirmed until work opens up the existing building. The right percentage depends on how developed the design is and how much uncertainty remains; your project team should explain what is included, excluded and provisional.
When comparing quotations, check the scope line by line. A low figure is not useful if it omits steelwork, drainage, finishes or making good the existing rooms.
3. Is this the right design for the house — not just the largest one?
Permitted development limits and planning policies define what may be possible, but they do not tell you what will make a good home.
A rear extension can add depth; a side-return extension can reclaim an awkward strip beside a Victorian kitchen; and a wrap-around extension can do both. A two-storey extension may create a bedroom or bathroom above new ground-floor space. The best option depends on the existing plan, garden, orientation, neighbouring homes and your priorities.
Ask what the extension will do to:
- daylight in the middle of the original house;
- the amount and usefulness of the garden;
- routes between the front door, kitchen and outside;
- privacy and views;
- storage and furniture placement;
- summer overheating and ventilation.
Do not judge the design only from a floor plan. Sections and simple three-dimensional views help explain ceiling heights, rooflights and the relationship between old and new. A large glazed wall may look impressive, but orientation, shading and heat loss need to be considered before glazing is specified.
4. What permissions and technical approvals will I need?
Planning permission is only one part of the approvals process.
Some house extensions in England fall under permitted development rights, provided they meet detailed limits and conditions. Larger single-storey rear extensions may require prior approval. Rights can be restricted for listed buildings, properties in conservation areas, homes affected by an Article 4 direction or houses whose original planning permission removed them.
If you intend to rely on permitted development, a lawful development certificate can provide formal confirmation that the proposal is lawful. Do not assume that a similar extension next door means yours automatically qualifies.
Building Regulations are separate and usually apply even when planning permission is not needed. They cover matters including structure, fire safety, insulation, ventilation, drainage and glazing. Your project may also need structural calculations, a build-over agreement for a public sewer, freeholder consent or restrictive-covenant approval.
Map the approvals before finalising the programme. The right route depends on the property and proposal, so confirm it with the local authority and relevant professionals rather than relying on a generic online summary.
5. How will the project affect neighbours, boundaries and shared walls?
Your neighbours do not decide every planning application, but their amenity and legal rights matter. A thoughtful design should consider loss of daylight, outlook, privacy, noise and the visual impact of the new building.
The Party Wall etc. Act 1996 may apply if work involves a shared wall, construction at or near the boundary, or excavation below the level of a neighbour’s foundations. It is a separate process from planning permission. If notices are required, statutory timeframes can affect the start date.
Check boundaries rather than relying on the position of an old fence. Confirm who owns or maintains walls, fences and trees, and identify any access you may need for scaffolding or external finishing.
Speak to neighbours early, once you have enough information to explain the proposal clearly. Show them the drawings, likely programme and how dust, noise and deliveries will be managed. A calm conversation will not remove every concern, but surprises tend to create more conflict than the work itself.
6. How long will the whole project take, and can we live at home?
People often ask how many weeks the build will take. The more useful question is how long the entire route from first design to handover will take.
Allow time for surveys, design decisions, cost planning, planning or permitted-development confirmation, technical drawings, structural engineering, Party Wall matters, contractor mobilisation and ordering long-lead items. Only then does construction begin.
As a current Dali Pro Services guide, a single-storey extension typically takes 12–16 weeks to build, while a double-storey project may take 16–22 weeks. These ranges describe the construction phase and will vary with scope, access, specification and unforeseen conditions.
Ask for a programme with decision dates, not just a completion date. Late choices about glazing, kitchens or finishes can delay work even when the site itself is well managed.
Whether you can remain at home depends on the work. Consider:
- when the existing kitchen will be removed;
- how long water, heating or electricity may be interrupted;
- whether bedrooms and the bathroom can be kept separate from the site;
- how dust and security will be controlled;
- whether children, pets or home working make occupation impractical.
A temporary kitchen may be enough for part of the build. For the most disruptive structural stages, a short stay elsewhere may be worth including in the plan and budget.
7. Who will design and build it — and how will I compare them?
Do not choose a team on price or polished photographs alone. Look for evidence that they can manage your type of property, the intended level of finish and the constraints of a London site.
Ask potential designers and builders:
- Who will be responsible for design, approvals, structural coordination and construction?
- Is the quotation fixed and itemised, and what is excluded?
- Which allowances are provisional?
- Who will manage the site each day and report progress?
- Can I visit a completed project and speak to a recent client?
- What insurance, accreditations and warranties are provided?
- How are changes priced and approved?
- What happens at snagging and handover?
If design and construction are handled by separate businesses, clarify where one party’s responsibility ends and the other begins. An integrated design-and-build route can reduce those gaps, but it still requires a transparent scope, clear drawings and an agreed change-control process.
Trust matters because you will work with the team for months. Clear communication before appointment is usually a good indication of how questions and problems will be handled during the build.
Your answers become the project brief
You do not need to know exactly what the extension should look like before speaking to a professional. You should, however, understand the problem you want to solve, the total amount you can invest and the compromises you are unwilling to make.
The first feasibility conversation should test those priorities against the property. It should identify the likely planning route, major technical constraints, realistic cost range and whether an extension is actually the best answer.
At Dali Pro Services, we design and build house extensions across South West London and Surrey. We can assess the house, discuss your priorities and outline a practical route from first sketch to handover.
**Arrange a free site visit or get an initial extension estimate.**
Frequently asked questions
What should I do first when planning a house extension?
Define the problem the project must solve and set a realistic total budget. Then arrange a feasibility assessment covering the existing house, planning constraints, access, structure and drainage before developing a detailed design.
Do I need an architect before speaking to a builder?
Not necessarily. Some clients appoint an independent designer first; others choose a design-and-build company that coordinates design, approvals and construction. Whichever route you take, the proposal needs enough detail for prices to be compared fairly.
How much contingency should I keep?
There is no single percentage suitable for every project. The amount should reflect the condition of the existing building, the completeness of surveys and design, and the number of provisional items in the quotation. Ask the project team to identify uncertainties rather than adding an unexplained allowance.
Does permitted development mean I can start building immediately?
No. The proposal must meet all relevant permitted-development limits and conditions, and some projects require prior approval. Building Regulations and other consents may still apply. Formal confirmation through a lawful development certificate is often sensible.
