How Much Does a House Extension Cost in London?
Understanding the Real Cost Before You Start
The cost of extending a home in London can vary enormously.
​
A modest rear extension to a straightforward property may be relatively easy to price. A larger wraparound extension involving structural alterations, extensive glazing, a new kitchen and limited site access can cost considerably more.
​
This is why homeowners should be cautious when they see an extension advertised at a single headline figure.
There is no universal price for a house extension.
​
The final cost depends on the size of the project, the type of property, the condition of the existing house, the design, the specification and the amount of work required beyond the new extension itself.
​
A realistic budget should consider the whole project rather than the cost of building the external shell alone.
A Useful Starting Point
As a broad working guide, many London house extensions fall within the following range:
​
Type of work - Indicative London construction cost
​
Straightforward extension with a sensible specification - £2,500–£3,200 per m²
​
Typical London extension with internal alterations and better-quality finishes - £3,200–£4,500 per m²
​
More complex or higher-specification project - £4,500+ per m²
​
These figures should be treated as an initial guide rather than a quotation.
​
The cost per square metre may rise where the extension is small, the access is difficult or the project involves substantial alterations to the existing house.
​
It is also essential to check whether VAT is included.
Example Extension Budgets
The table below provides a practical starting point for early budgeting.
​
Example project - Indicative construction budget
​
Small rear extension of around 15–20 m² - £45,000–£80,000
​
Typical rear extension of around 20–30 m² - £65,000–£120,000
​
Side-return extension with internal alterations - £60,000–£120,000
​
Wraparound extension of around 30–40 m² - £95,000–£170,000+
​
Larger or higher-specification extension - £150,000+
​
Double-storey extensionHighly project-specific, but often better value per m² than a single-storey addition
The figures can increase substantially where the project includes a new kitchen, extensive glazing, major structural work, drainage alterations or a higher level of finish.
​
A side-return extension may add fewer square metres than a conventional rear extension but still involve significant cost. Walls may need to be removed, structural steelwork introduced and the existing kitchen completely replaced.
​
​
Why Small Extensions Can Be Expensive Per Square Metre
It is natural to assume that a small extension will always be inexpensive.
​
That is not necessarily the case.
​
Every extension requires foundations, walls, insulation, drainage, structural work, a roof, glazing, electrics, heating, plastering and decoration.
​
Many of these costs apply whether the extension adds 12 square metres or 30 square metres.
​
A smaller project may therefore cost less overall but more per square metre.
​
This is particularly relevant for side-return extensions, where the new floor area may be modest but the work can involve substantial alterations to the existing house.
​
​
Why London Extensions Cost More
Building in London is rarely straightforward.
​
Labour costs are generally higher than in many other parts of the country. Parking restrictions, congestion, narrow streets and limited storage can make deliveries more difficult. Waste removal, skips and scaffolding may need to be organised carefully.
​
​
Many London homes are terraced or semi-detached period properties. Access to the rear garden may be limited, meaning that materials need to be carried through the house or brought in through neighbouring land by agreement.
​
The existing property may also conceal complications.
​
Older drains, shallow foundations, awkward levels, previous alterations and poorly supported walls can all increase the cost once work begins.
​
The cheapest quotation is not always the most realistic one.
​
​
The Type of Extension Matters
Rear Extensions
A straightforward single-storey rear extension is usually the easiest type of extension to budget for.
​
Costs will still vary depending on the depth, width, roof design, glazing and the amount of internal alteration required.
​
A simple rectangular extension with standard rooflights and sensible doors will usually cost less than a highly glazed design with bespoke detailing.
​
​
Side-Return Extensions
A side-return extension fills the passage beside the original rear addition of a Victorian or Edwardian house.
​
Although the additional space may be relatively modest, these projects can be expensive per square metre.
​
The work often involves structural steelwork, alterations to the existing kitchen, new drainage and careful roof design to bring natural light into the centre of the home.
​
​
Wraparound Extensions
A wraparound extension combines a side-return extension with a rear extension.
​
This can create a generous kitchen, dining and family area, but the budget needs to reflect the increased structural work and larger internal fit-out.
​
A wraparound extension may also require more complicated steelwork, drainage alterations and glazing.
​
​
Side Extensions
A side extension can be a practical option for semi-detached and detached homes.
​
The cost will depend on the intended use. A simple utility room or home office may be relatively straightforward.
​
A bedroom with an en-suite bathroom or a more flexible living space will usually require a higher budget.
​
​
Double-Storey Extensions
A double-storey extension can offer better value per square metre than a single-storey extension.
​
The foundations and roof are among the more expensive elements of any extension. Adding a first floor above the same footprint can create valuable additional space without doubling every part of the build cost.
​
However, the design may require more careful planning, additional structural work and alterations to the upstairs layout.
​
​
Extensions and Loft Conversions
Some homeowners choose to combine a ground-floor extension with a loft conversion.
​
This can be an efficient way to improve the whole house, particularly where a family needs a larger kitchen downstairs and an additional bedroom or bathroom upstairs.
​
The overall budget will be higher, but coordinating the work may reduce duplication and avoid completing one major project only to disrupt the house again a year later.
Construction Cost Is Not the Same as the Total Project Cost
One of the most common budgeting mistakes is to focus only on the builder’s quotation.
A realistic project budget may also need to allow for:
-
design drawings and planning support;
-
planning application or Lawful Development Certificate fees;
-
Building Regulations drawings;
-
structural calculations;
-
Building Control or Registered Building Control Approver fees;
-
party wall advice;
-
Thames Water or drainage-related requirements;
-
specialist surveys where required;
-
kitchen supply and installation;
-
glazing, rooflights and external doors;
-
flooring and decoration;
-
landscaping and patios;
-
furniture, lighting and window treatments;
-
temporary accommodation or storage;
-
contingency funds.
Not every project needs every item.
However, it is important to identify the likely extras before accepting a builder’s quotation.
VAT Can Change the Budget Significantly
VAT is often overlooked during early conversations.
Most extension work to an existing home is subject to VAT at the standard rate.
A quotation of £80,000 excluding VAT becomes £96,000 once VAT is added.
A quotation of £120,000 excluding VAT becomes £144,000.
Always ask whether VAT is included and whether the contractor is VAT registered.
Two builder quotations can appear very different simply because one includes VAT and the other does not.
Professional Fees
Professional fees should be viewed as part of the project rather than an unnecessary extra.
The right drawings and advice can help homeowners avoid expensive mistakes, obtain suitable planning approval and compare builder quotations more accurately.
Depending on the project, professional support may include:
Design and Planning Drawings
These establish the proposed layout, appearance and planning route.
They allow the homeowner to explore options before committing to construction.
Building Regulations Drawings
These deal with the technical aspects of the project, including insulation, drainage, ventilation, roof construction, fire safety and damp protection.
Structural Calculations
A structural engineer may be required where walls are removed, steel beams are introduced or the design involves more complex structural changes.
Building Control
Most extensions require Building Regulations approval, even where planning permission is not required.
Building Control or a Registered Building Control Approver will review the technical information and inspect the work during construction.
Party Wall Advice
Terraced and semi-detached homes frequently require early consideration of the Party Wall etc. Act 1996.
The process is separate from planning permission and Building Regulations approval.
The Kitchen Can Transform the Cost
Many extensions are built to create a larger kitchen and dining area.
The kitchen can become one of the largest individual cost items within the project.
A sensible kitchen with standard appliances may fit comfortably within a controlled budget. A bespoke kitchen with stone worktops, premium appliances, extensive joinery and specialist lighting can add tens of thousands of pounds.
The important point is not that homeowners should always choose the cheapest kitchen.
It is that the kitchen budget should be separated clearly from the structural build cost so that it does not become an unexpected surprise later.
Glazing, Rooflights and Doors
Large sliding doors, aluminium-framed glazing, roof lanterns and rooflights can transform an extension.
They can also increase the cost substantially.
The design should balance appearance, natural light, energy efficiency and budget.
A large wall of glazing is not always the best answer. In some homes, carefully positioned doors and rooflights create a brighter and more attractive space at a lower cost.
Glazing should be considered during the design stage rather than selected hurriedly once construction is underway.
Structural Alterations
Removing walls can make an extension feel much more open and useful.
However, structural alterations need to be priced realistically.
The cost may include:
-
steel beams;
-
padstones;
-
columns;
-
temporary supports;
-
making good;
-
changes to ceilings and floors;
-
alterations to electrics and plumbing;
-
structural calculations;
-
additional Building Control inspections.
An apparently simple decision to remove a wall can affect several parts of the build.
This does not mean the wall should remain. It means the cost should be understood before construction begins.
Drainage and Ground Conditions
Drainage can have a significant effect on the budget.
​
An extension may be built over or close to existing drains. Inspection chambers may need to be relocated. A build-over agreement may be required where a public sewer is affected.
​
Ground conditions also matter.
​
Clay soil, mature trees, changes in level and previous extensions can affect the foundation design.
​
These issues are not always visible during the earliest design stage, which is why a sensible contingency is important.
​
​
Access and Logistics
London access problems should not be underestimated.
​
A project on a wide suburban plot with side access will usually be easier to organise than a rear extension to a terraced house where every bag of cement and length of steel needs to pass through the property.
​
The budget may be affected by:
​
-
parking suspensions;
-
skip permits;
-
scaffolding;
-
restricted delivery times;
-
manual handling;
-
waste removal;
-
limited storage;
-
protecting existing floors and walls;
-
temporary kitchen arrangements.
​
A builder familiar with London projects should consider these practical issues before providing a final quotation.
​
​
Hidden Extras
Many cost overruns are not caused by one dramatic problem.
​
They come from a series of smaller items that were not properly considered at the outset.
​
Common examples include:
​
-
replacing an outdated boiler;
-
upgrading the electrical consumer unit;
-
moving radiators;
-
changing floor levels;
-
repairing damaged garden areas;
-
replacing fences;
-
redecorating rooms affected by the work;
-
temporary storage;
-
additional lighting;
-
fitted cupboards;
-
utility-room joinery;
-
external taps and sockets;
-
blinds for large glazed doors;
-
additional waste removal.
​
A detailed budget should include a section for these secondary items.
​
​
How Much Contingency Should You Allow?
Every extension project should include a contingency fund.
​
A sensible starting point is usually around 10% of the construction budget.
​
A higher allowance may be appropriate where:
​
-
the property is older;
-
access is difficult;
-
drainage is uncertain;
-
substantial walls are being removed;
-
earlier alterations are poorly documented;
-
the specification is still evolving;
-
the homeowner is likely to make changes during construction.
​
A contingency is not an invitation to spend more.
​
It is protection against the unexpected.
​
​
How to Keep the Budget Under Control
The most effective cost-saving decisions are usually made before building work starts.
​
​
Resolve the Layout Early
Changes made on paper are inexpensive.
​
Changes made after foundations, drainage or structural steelwork have been installed can be costly.
​
​
Avoid Building More Space Than You Need
A larger extension is not always a better extension.
​
Good natural light, practical storage and a thoughtful layout can create more value than additional floor area alone.
​
​
Obtain Comparable Builder Quotations
Builders should price the same drawings and the same general specification.
​
This makes it easier to understand whether a quotation is genuinely competitive.
​
​
Identify Exclusions
Ask each builder to state clearly what is not included.
​
A low quotation may simply leave more work to be priced later.
​
​
Separate Essentials from Upgrades
The structural work, insulation, drainage and weatherproofing are essential.
​
Premium kitchen appliances, bespoke joinery and higher-end finishes can be adjusted if the budget becomes stretched.
​
​
Avoid Continual Changes During Construction
Variations are one of the easiest ways for costs to rise.
​
Spend enough time on the design before work begins.
​
​
Comparing Builder Quotations Properly
A builder should not be selected on headline price alone.
​
Before appointing a contractor, check whether the quotation includes:
​
-
demolition and site preparation;
-
foundations and drainage;
-
structural steelwork;
-
roof construction;
-
insulation;
-
windows, doors and rooflights;
-
plumbing and electrics;
-
heating alterations;
-
plastering and decoration;
-
flooring;
-
kitchen installation;
-
waste removal;
-
scaffolding;
-
parking or access costs;
-
Building Control requirements;
-
VAT;
-
the treatment of variations.
​
Where one quotation is substantially lower than the others, the homeowner should ask why.
​
The missing cost may not disappear. It may simply emerge later.
​
​
Is an Extension Worth the Cost?
A house extension should not be judged on cost alone.
​
It may allow a family to remain in an area they enjoy, avoid the expense of moving and create a home that works better for everyday life.
​
In higher-value parts of London, a well-designed extension may also add substantial value to the property.
​
However, value is not created simply by adding square metres.
​
The strongest projects improve the whole home.
​
They create better natural light, stronger garden connections, more practical storage and a layout that feels balanced.
​
A poorly designed extension can be expensive without making the property significantly better.
​
​
Start with a Realistic Budget
The best starting point is not to ask how cheaply an extension can be built.
​
It is to decide what you need the project to achieve and how much you are realistically comfortable spending.
​
Once the broad budget is understood, the design can be developed around it.
​
A modest and carefully planned extension will often deliver a better result than an ambitious scheme that becomes financially uncomfortable halfway through construction.
​
​
Planning a House Extension in London?
Tell us a little about your home, the type of extension you are considering and the stage you have reached.
APN Extensions can help you explore suitable options and identify the professional support that may be useful for your project.