October 29, 2026

House Extension Costs in the UK: Prices by Type and Budgeting Guide

A house extension can create a larger kitchen, additional bedrooms or more flexible living space without moving home. The main budgeting challenge is understanding what the quoted price covers, rather than simply finding a cost per square metre.


As an initial guide, UK extension projects can range from around £30,000 to £150,000 or more. A typical single-storey extension may cost £40,000 to £70,000, while a substantial basement project can exceed £200,000 in London.


Your final cost depends on the extension type, floor area, location, materials and design complexity. Professional fees, approvals and VAT also need to be accounted for. The figures below are starting points for planning, not fixed prices or a substitute for project-specific quotes.


Key Takeaways

  • A typical single-storey extension costs around £40,000 to £70,000, with higher prices in expensive locations.
  • Two-storey extensions can cost less per square metre because both floors share the roof and foundations.
  • Budget separately for professional fees, approvals and VAT unless they are explicitly included.
  • Use local quotes for the same design and specification to establish a realistic total.



Table of Contents

How Much Does a House Extension Cost?


The following comparison shows indicative construction costs for common ways to add space. Before using any figure in your budget, establish whether VAT and professional fees are included.


These ranges are not directly interchangeable. A typical project total and a square-metre rate may reflect different sizes and specifications. Use the rate to explore your proposed floor area, then obtain quotes for the actual design.


How to calculate a preliminary construction budget

A simple starting calculation is:


Added floor area × indicative cost per square metre = preliminary construction estimate.


For example, a 40-square-metre wraparound extension at £2,000 to £3,500 per square metre gives a construction range of £80,000 to £140,000. That does not automatically make £140,000 your maximum overall budget: fees, approvals and VAT may still need to be added.


For a two-storey house extension, calculate the combined added floor area across both floors. A footprint of 20 square metres with an equally sized upper floor provides 40 square metres in total.

Choose the House Extension That Solves Your Space Problem


The most useful comparison is not simply which option costs least. It is which approach creates the rooms you need at a cost you can support.


For a bigger kitchen or living area

A single-storey extension is a common choice for enlarging a kitchen, adding dining space or creating an open-plan layout. It keeps the added accommodation on one level.


A wraparound house extension combines side and rear additions into an L-shaped layout. It can provide more floor area and greater scope to reshape ground-floor living space, but its price depends heavily on complexity and structural work.


For bedrooms as well as downstairs space

A two-storey extension can add bedrooms upstairs while expanding the kitchen or dining space below. Its cost per square metre is often lower than that of a single-storey addition because the roof and foundations serve both floors.


That does not mean a two-storey house extension has a lower total cost. It means the extra floor area can make better use of shared construction elements.


For extra space without expanding the footprint

A loft conversion can provide a bedroom or home office without extending outwards. It is worth comparing with an external addition when your main need is another room rather than a larger ground floor.


A basement conversion is another option when building upwards or outwards is not possible. However, it sits towards the expensive end of the cost range and needs careful budgeting.

What Makes a House Extension More Expensive?


Location

Costs in London and the South East are generally higher than in other parts of the UK. A single-storey project costing around £40,000 in one region may exceed £80,000 in London.


For a realistic house extension budget, use local quotations rather than assuming a national average applies to your property.


Size

A larger extension generally costs more overall. Square-metre pricing makes this relationship easier to assess, but it remains a broad estimating tool rather than a fixed tariff.


Be clear about how much added space you need before comparing prices. A lower rate applied to a much larger floor area can still produce a higher total.


Materials and finishes

Standard materials such as brick and concrete generally cost less than premium choices involving extensive glass or steel. A sleek contemporary finish or particular environmentally focused options can also increase the budget.


Compare the same specification. A quote for a straightforward build should not be treated as equivalent to one featuring premium materials and bespoke details.


Design and structural complexity

Custom-made elements, intricate designs and substantial structural work increase costs. They can require specialist labour as well as more expensive materials.


If affordability is a priority, assess whether a simpler house extension could deliver the same practical benefit before committing to an elaborate design.

Costs to Add Beyond the Building Work


A construction estimate is only one part of a complete house extension budget. Check the following items separately unless the quotation explicitly includes them.


Architect fees

Architect fees typically range from 5% to 15% of the project cost. On a £50,000 cost base, that equates to approximately £2,500 to £7,500.


Ask which services the fee covers and which cost figure is used to calculate it. A percentage is only useful when its basis and scope are clear.


Structural engineering

Structural calculations are another expense to allow for. Obtain a project-specific fee rather than assuming this work is included in the architect’s appointment or builder’s quotation.


Planning application fees

If your project requires a planning application, include the relevant application fee in your budget. Confirm the current charge with the local planning authority rather than relying on an older estimate.


Building control

A house extension needs building control approval, with inspections during construction. An indicative allowance is £600 to £850, but confirm the actual charge for your scheme.


Keep planning and building control costs as separate budget items. They are distinct parts of the approval process.


VAT

Most building projects are subject to 20% VAT. Always establish whether a price includes it.


For example, building work quoted at £50,000 excluding VAT becomes £60,000 after adding VAT at 20%. That £10,000 difference can materially change whether the project is affordable.

How to Build a Realistic House Extension Budget


  1. Define the rooms you need. Decide whether the priority is a kitchen, bedrooms, an office or a combination.
  2. Compare suitable project types. Consider whether a single-storey, two-storey, wraparound, loft or basement solution best meets that need.
  3. Estimate the added floor area. Use total floor area across all added levels, not just the ground-floor footprint.
  4. Calculate an initial construction range. Apply the relevant square-metre figures as a starting point.
  5. Account for location and specification. Revisit the estimate if the project involves a high-cost region, premium materials or bespoke work.
  6. Add fees and approvals. List architect, engineering, planning and building control costs separately.
  7. Clarify VAT. Mark every quotation as VAT-inclusive or VAT-exclusive.
  8. Get multiple quotes for the same brief. Compare what each price includes before choosing a contractor.


The aim is to move from a broad house extension estimate to a clear, itemised budget before committing to construction.

Common House Extension Budgeting Mistakes


  • Treating a square-metre rate as an all-in price. It may not include professional fees, approvals or VAT.
  • Using the wrong area for a two-storey project. Both floors count towards the total added floor area.
  • Comparing unlike quotations. Different materials, finishes and scopes can explain substantial price differences.
  • Applying regional prices to London. Location can change the budget significantly.
  • Assuming a lower unit cost means a cheaper project. A two-storey addition may cost less per square metre while costing more overall.
  • Choosing bespoke features before checking affordability. Custom details and specialist labour can raise costs quickly.

Start With the Space You Need, Then Confirm the Full Cost


A successful house extension budget connects three things: the rooms you need, a suitable design and a complete cost breakdown. Start with indicative ranges, then replace assumptions with local quotations and confirmed fees.


Before proceeding, make sure you can explain what the construction price covers, which costs sit outside it and whether VAT is included. That is more useful than relying on a single headline figure.

Frequently Asked Questions

How much does a 40-square-metre house extension cost?

Using the indicative rates above, 40 square metres of total added floor area costs approximately £60,000 to £100,000 for a two-storey extension or £80,000 to £140,000 for a wraparound extension. Confirm whether fees and VAT are included before treating either range as an overall budget.

Is a two-storey extension cheaper than a single-storey extension?

A two-storey extension often has a lower cost per square metre because its roof and foundations serve two floors. It is not necessarily cheaper overall, since it creates more floor area.

Why does a house extension cost more in London?

Location is a significant cost factor, with London and the South East generally more expensive than other UK regions. Local quotations are therefore a better budgeting basis than national averages alone.

Should VAT be included in my extension budget?

Yes. Most building projects attract 20% VAT, so check whether each quotation includes it. A £50,000 building quote excluding VAT becomes £60,000 when VAT is added at 20%.

PREVIOUSNEXT
Top Trends in Commercial Architecture: What to Expect in 2023

Join our Newsletter

Subscribe to our monthly newsletter and get updates and industrial insights delivered to your inbox.

Keep Me Updated

Back to top