Can the extension you want actually be built properly, within planning rules, site conditions and a realistic budget?
Many homeowners start with drawings. That makes sense. A good architect can help shape the design and guide the planning process. But design is only one part of the project. A house extension also has to work financially, structurally and practically on site.
That is why it is worth speaking to an experienced building company early, not only once the drawings are finished.
Start with a realistic budget, not just drawings
A well-designed extension should look good, but it also needs to be buildable.
Sometimes a design looks impressive on paper but becomes much more expensive once the real construction details are considered. Structural steel, drainage, access, ground conditions, roof design, glazing, kitchen specification and finishes can all change the final budget significantly.
In the UK, single-storey extensions are often quoted around £1,800 to £3,000 per square metre, while London and the South East are usually higher because of labour costs, restricted access, parking, planning complexity and site constraints. These figures are useful as a starting point, but they are not enough to plan a real project.
A 25 m² extension is not simply “25 times a square metre rate”. The final cost depends on what is included, what is excluded and what level of finish you expect at the end.
If you want a quick starting point, you can also use our interactive Extension Cost Calculator. It gives you a realistic cost range for an extension, loft conversion or new build in London and Surrey, based on your property type, project size, planning situation and key design choices.
Professional fees come before the building work
Before construction starts, there are usually several professional and statutory costs to allow for.
Depending on the project, you may need:
- architect or designer fees
- measured surveys
- planning drawings
- planning application fees
- structural engineer calculations
- building regulations drawings
- building control fees
- party wall surveyor fees
- sometimes additional reports or consultant input
Some home extensions can be built under permitted development rights, provided they meet the relevant limits and conditions. If the design goes beyond those limits, a householder planning application is usually needed.
The current householder planning application fee in England is £548. This is only the application fee. It does not include the cost of preparing drawings, managing the planning process or making design changes if they are needed.
Building regulations are separate from planning permission. Even if your extension does not need full planning permission, most extensions still require building regulations approval.
Party wall matters can also add cost and time, especially in terraced or semi-detached homes. In most cases, the building owner carrying out the work pays the costs associated with the Party Wall Award, including reasonable surveyor fees where required.
When all of these items are combined, it is not unusual for homeowners to spend several thousand pounds before any construction work starts. On more involved projects, the pre-build stage can easily approach £10,000 or more.
Hidden site costs: scaffolding, waste and setup
One area homeowners often underestimate is the cost of preparing and running the site.
Before the extension itself can be built, the contractor may need to organise:
- scaffolding
- site protection
- hoarding or temporary fencing
- welfare facilities
- temporary toilet
- storage space for tools and materials
- deliveries
- waste management
- general site setup and site management
These costs are not optional extras. They are part of running a safe, organised and efficient building site.
Even on smaller extensions, preliminary costs can quickly reach several thousand pounds. On more complex sites, especially where access is difficult or scaffolding is extensive, they can be much higher. Waste disposal is another major cost that is easy to underestimate. Excavation, demolition, old patios, soil, rubble, packaging and general construction waste all have to be removed properly. This usually involves skips, grab lorries or weight-and-load collections.
A simple extension with good access and limited excavation may only need a modest allowance. But if there is a lot of digging, poor access, heavy soil removal or demolition, waste costs can increase quickly. For many projects, waste disposal can run into several thousand pounds. On larger or more difficult jobs, it can become a five-figure cost.
Scaffolding is similar. A basic scaffold for a smaller extension might cost a few thousand pounds. More complex scaffolding, such as temporary roof structures for loft conversions or projects that need weather protection, can cost significantly more.
This is one of the reasons why a cheaper quote is not always a better quote. Sometimes important site costs are missing, unclear or underestimated. That can lead to extra charges later.
The build itself: foundations, steelwork and structure
Once the project starts, the main construction phase usually includes:
- excavation
- foundations
- drainage alterations
- concrete works
- steelwork
- brickwork or blockwork
- roof structure
- insulation
- windows and doors
- making the extension watertight
This stage is obviously important and can be expensive. But for experienced builders, it is often the more straightforward part of the job because the process is familiar and clearly defined.
The real risk is when the structural part has been priced, but the rest of the project has not been thought through properly.
A house extension is not finished when the walls and roof are built. It is finished when the space works as part of your home.
Finishes are where budgets change the most
This is where many homeowners underestimate the cost.
Internal finishes can include:
- plastering
- flooring
- decorating
- electrics
- lighting
- plumbing
- heating
- kitchen installation
- bathroom or shower room
- tiling
- internal doors
- skirting and architraves
- sanitary ware
- appliances
- built-in furniture
This is also where specification makes a huge difference. A kitchen might start from around £15,000, but it can easily reach £50,000 or more depending on the design, appliances, worktops, cabinetry and installation complexity.
The same applies to bathrooms, tiles, taps, showers, toilets, flooring and lighting. You can buy a very cheap toilet, tap or tile. But price is not the only question. You also need to think about quality, reliability, spare parts, finish, warranty and how long the product will last.
For example, a toilet for £150 and a toilet for £400 may seem like the same item when you are trying to reduce the budget. But the difference can show later in the flush mechanism, finish, durability, parts availability and overall reliability.
The same principle applies across the whole project. Cheaper products are not always wrong. In some areas, they may be perfectly suitable. But they should be chosen knowingly, not just because they reduce the first quote.
A proper budget should separate essential construction costs from specification choices. That way, you can make informed decisions rather than discovering halfway through the project that the finish you want was never really included.
Do not forget the outside
Landscaping is another cost that is often forgotten. Many homeowners say they will deal with the garden later. The problem is that “later” often means after the construction budget has already been spent. The result can be a beautiful new extension opening onto a muddy, unfinished garden for months or even years.
External works can include:
- patios
- porcelain paving
- drainage
- steps
- retaining walls
- fencing
- turf
- planting
- outdoor lighting
- garden paths
These items add up quickly. They also affect how the finished extension feels. A rear extension is usually designed to connect the house with the garden, so the outside space should be considered from the beginning.
You do not always need to complete everything at once. But you should at least understand what the finished external works are likely to cost before the main build starts.
Why speaking to an experienced builder early matters
A lot of builders can build. Fewer can manage the whole project properly.
A good extension involves coordination between many people:
- architects
- structural engineers
- building control
- party wall surveyors
- scaffolders
- waste companies
- plumbers
- electricians
- roofers
- kitchen suppliers
- decorators
- landscapers
As a homeowner, trying to organise all of this yourself can quickly become stressful. An experienced building company should already have trusted contacts, understand the order of works and know how to deal with problems when they happen on site.
This is especially important in South West London and Surrey, where access, neighbouring properties, parking, deliveries and site restrictions can make even a simple extension more complicated.
Good advice at the beginning can help you understand what is realistic, where the budget is likely to move and which decisions will have the biggest impact on the final cost.
What should your extension budget include?
To get a clearer idea of where your project might sit, try our free Extension Cost Calculator before you start comparing quotes.
A realistic house extension budget should usually include:
- Professional fees and applicationsDesign, planning, structural engineering, building regulations, building control and party wall matters.
- Preliminaries and site setupScaffolding, welfare, site protection, access, storage, waste planning and site management.
- Core constructionFoundations, drainage, structural work, walls, roof, insulation, windows and doors.
- ServicesPlumbing, heating, electrics, lighting, ventilation and any upgrades needed to existing systems.
- Internal finishesFlooring, decorating, kitchen, bathrooms, tiling, doors, skirting, sanitary ware and appliances.
- External worksPatios, garden levels, drainage, fencing, steps, turf, paving and lighting.
- ContingencyA sensible contingency is important because existing homes can hide problems until work begins.
Final advice before starting your extension
Before committing to a house extension, do not only ask:
“How much per square metre?”
A better question is:
“What is included, what is not included, and what will it cost to finish the project properly?”
The cheapest quote can become expensive if important items are missing. A realistic quote should help you understand the full journey from planning and site setup through to finishes and outside works.
At Dali Pro Services, we believe good planning at the beginning saves stress, delays and disappointment later. Before starting a project, we sit down with clients and talk through what is realistic, what is important, where they can save and where it is worth investing more.
If you are thinking about extending your home in South West London or Surrey, start with a realistic budget. Use our free Extension Cost Calculator to get an instant estimate, then speak to Dali Pro Services before you commit to drawings or a final quote.
We can help you understand what is realistic, what to allow for and where your money is best spent.
FAQ
How much does a house extension cost in London?
As a broad guide, many UK single-storey extensions are quoted around £1,800 to £3,000 per square metre, with London often sitting higher depending on size, specification, access and complexity. The final cost depends on what is included in the quote and what level of finish you want.
Do I need planning permission for a house extension?
Not always. Some extensions fall under permitted development rights if they meet the relevant limits and conditions. If the design goes beyond those limits, a householder planning application is usually required.
Do I still need building regulations approval?
In most cases, yes. Planning permission and building regulations are separate. Most extensions require building regulations approval even if full planning permission is not needed.
What costs are most often forgotten?
Commonly underestimated costs include professional fees, party wall matters, scaffolding, waste disposal, site setup, internal finishes, kitchens, bathrooms and landscaping.
Should I speak to a builder before an architect?
It is often a good idea to speak to both early. An architect can help with design and planning, while an experienced builder can advise on buildability, realistic costs, site constraints and where the budget is likely to change.
