“How much will my house extension cost?”


award winning architects sw london

There is no quick answer to this and you will never truly know how much your house extension costs until you have a full set of plans and a detailed Scope of Works drawn up with 3-4 tender quotes from contractors!

But that’s not all…

You will need to research and choose a new kitchen, bathroom finishes, sanitary ware as well as many more variables.

You may already have to pay £000’s to architects and design consultants and hours of time to get to that stage.

We would always advocate paying a Quantity Surveyor (QS)/Cost consultant to produce a cost estimate, but….

Is there a better way for free?

Yes, and we’ll explain more in this article.

House extension cost breakdown

This guidance is by no means definitive, but it will give you a ball park figure in which to help you move forward.

Let’s start by saying that this article is aimed mainly towards projects in London and the South East. Labour costs and some materials will vary depending on where you are in the country but we can only advise on our patch!

Those of you outside London and the South East will still benefit from reading this and we’re sure you will gain some valuable tips on what your house extension costs.

Later in the article we will show some real-world examples to help you piece together the information.

Costs overview – Estimates

The table below gives an overview and estimated price range for how much your house extensions may cost.

Building Element
New Extension (single storey)
£1,500 – £2,500
Per m2 additional floor area

The lower range would be a basic construction with uPVC windows/doors. The higher range would be a more complex construction with specialist glazing. Note that small extensions will likely have a higher £/m2 rate.

New Extension (additional storey)
£1,000 – £1,500
Per m2 additional floor area

When building a second storey you no longer have the groundwork costs which can be considerable

Major Structural opening (streel frame)
£3,000 – £10,000
Per opening

When creating an open-plan kitchen/living area for example, this cost would be relevant to removing the existing ground floor rear wall of the house.

Loft Conversion
£1,500 – £2,000
Per m2 of additional floor space (above 1500mm ceiling height)

The cost of this will rise with the complexity of your existing roof structure. If your roof and proposed layout is complex it may be worth considering removing the existing structure completely and installing a new roof.

Basement (under existing house plus underpinning)
£2,250 – £3,000
Per m2 additional floor space

Site accessibility and logistics will play a large part in your basement extension hence the wide range in costing.

Basement (under garden)
£1,750 – £2,250
Per m2 additional floor space

This can be a great option in conjunction with walk-on rooflights to let light in to your new basement space.

Internal refurbishment (including minor structural works e.g. new door openings)
Up to £500 – Cosmetic
£500-£750 – New wiring/plumbing and back to brick£750-£1,000 – including layout changes and
structural work
Per m2 refurbished floor area

Back to brick? New wiring/plumbing? Or just a cosmetic uplift? This cost will vary greatly.

Removing chimneys
£2,000 – £5,000
Per chimney

Depending on height, new structural details, site access, ownership, type of construction.

New Bathroom (3-piece fully tiled)
£2.,500 Shell
Sanitary ware and tiles
£2,000 – basic£5,000
£10,000 – high-spec
Per bathroom

‘Shell’ would include carpentry, plumbing, electrics.


We have given an indication of how much you are likely to spend depending on level of finish.



£5,000+ – basic

£10,000+ – mid-range

£30,000+ – high-spec

Generic 20 kitchen units including appliances

Basic – e.g. Howdens/Magnet, basic appliance spec, laminate worktops
Mid-range – e.g. Wren/IKEA, better appliance spec (e.g. siemens), stone worktops
High-spec – bespoke/designer manufacturer, top
appliance range (e.g. Miele), stone worktop.

What will affect my house extension costs?

Drainage location and diversions

If you’re building over or near a Thames Water sewer that is classified as ‘Public’ you will likely need to submit relevant drawings and details to apply for a Thames Water Build-over agreement. We explain more about these in our blog.

Ground/Soil type

This will affect the type and depth of your foundations.  Either a soil investigation will be required, or your structural engineer will advise on the likely ground conditions.

Nearby Trees

Trees absorb ground water and can affect the shrinkability of the soil surrounding your house. This may require deeper foundations adding extra cost. Leylandii trees in a London clay soil close to your proposed extension can require 2.5m deep foundations and in certain scenarios make trench foundations unfeasible. Click here to find out which tree is in your garden and how deep your foundations may need to be using the handy NHBC Foundation Depth Calculator.

Property Age

Period and older properties will often bring unexpected costs and uncover hidden problems that were unforeseen. If you are carrying out building works on older properties ensure you have a healthy contingency in your budget and ensure you carry out a full structural survey of the property as early as possible.

Structural complexity

This is where a good Engineer can save you money. There will be many ways to design your supporting structure, and a good engineer will design a lean scheme with due regard to an easy installation on site and good detailing in conjunction with your architect.

Procurement method

This is another topic in itself which relates to the level of project management you wish to undertake yourself, the level of risk, your building contract, and many other factors which will form the topic of another useful article from Aura Homes.


Having a good relationship with your neighbours can keep your project moving swiftly especially when your works require Party Wall Agreements in compliance with the Party Wall Act 1996. More useful information on Party Wall Awards can be found here.

Health and Safety

You have obligations as the homeowner under the Construction Design and Management Regulations (CDM) 2015 – click to find out more. Your contractor will need to be aware of these as they will have the greatest responsibilities during your build project. Incorrect H&S on site can cause delays and even fines so ensure your contractor has sufficient cost in his tender to fulfil his duties.

and much more….

What Professional and other Statutory fees will I need to pay?

Fees – Architect

The traditional method for an architect to calculate their fee is based on a % of the building contract value.

They will make similar assumptions to that shown in the Cost Overview above to value your project from the outset. Their typical fee will be between 10-15% the total contract value (including all subcontracted work).

If there are variations during the project i.e. an additional en-suite is to be added, they would charge a % fee based on the additional cost of constructing the en-suite.

At Aura Homes we take a different approach!  See our Architect Fee Structure here.

We don’t believe that you should be paying more in professional fees because the level of finish and quality of your fittings raise the contract value of your project.

Realistically as designers we are still carrying out a similar level of work, so why should you pay extra?

Because of this philosophy, we provide a fixed fee by calculating how long we expect your project to take and multiply this by an hourly rate.

Fixed fee quote = Length of time to complete your brief x hourly rate

Our expected ‘time spent’ is based on similar past projects based on your initial design brief.

Fees – Full Measurement/ Topographical Survey

To ensure we are working to accurate, measured data, you will need to instruct a Surveyor to provide a full measurement survey of your property.

Measured Survey Measured Survey 2

This will include:

  • room dimensions
  • window locations and measurements
  • roof ridge height
  • floor levels
  • eaves heights
  • ground levels
  • and everything else we would need to work from.

The surveyor will provide us with a digital CAD file of the data so that we can start detailing your proposals.

Cost: £800+ (depending on size of property)


Fees – Structural Engineer

This will vary depending on the complexity of the design. Guide costs below (VAT not included):

Simple loft conversion or single storey rear extension£600-1000
Combination of the above, side and rear extension£1000-1500
Combination of the above plus internal structural alterations£1500-3000
Major works to 4+ bedroom property£3000+

Fees – Party Wall Surveyor

You will need to notify your neighbours of any relevant works being carried out under the Party Wall Act 1996.

More information can be found here but in general, if you’re installing foundations within 3m of your neighbours property, or putting steel beams within party walls you will need to notify your neighbour and potentially draw-up Party Wall Agreement.

Cost: To serve notice, circa. £200.  From £900 up to £2000+ (if an Award is needed)

Fees – Drainage CCTV survey

If you do not know the routes and flow direction of your property’s drainage we’d suggest paying for a CCTV survey.

This will give us the location and direction of drainage, manhole invert (depth) and cover levels, and detail whether there is any damage to the pipes.

Cost: £300-400

Fees – Thames Water Build-over Agreements

If you’re building over or near a Thames Water sewer that is classified as ‘Public’ you will likely need to submit relevant drawings and details to apply for a Build-over Agreement.

Most residential applications will be for pipes up to 160mm diameter. There will be additional work for us to generate the design documents and manage the application on your behalf.

Cost: £343+

Fees – Planning permission

Rates increased by 20% on 17th Jan 2018.

Householder Planning Application Fee now costs £206

Certificate of Lawful Development – £103

A full Planning Fee Schedule can be downloaded from the Planning Portal via the link.

Fees – Building control

We will generally recommend instructing a private Approved Building Control Body as opposed to the Local Authority Council (*our preference only).

A Building Control application is split in two sections.

  1. Plans Check

    This is where our drawings and the engineers calculations are submitted to the inspector and a desktop study carried out to check compliance with the Building Regulations Approved Documents. Payment for this is generally required on submission of the application.

  2. Inspection Checks

    These are carried out at various stages of the build to check progress on site. The inspector will check compliance with the Building Regulations and comment on any alterations needed. Payment for this is generally required following the first inspection.

Simple loft conversion or single storey rear extension£300 ex.VAT£350 ex.VAT
Combination of the above, side and rear extension£400 ex.VAT£500 ex.VAT
Combination of the above plus major internal structural alterations£500 ex.VAT£700 ex.VAT
Major works to 4+ bedroom property£600 ex.VAT£900 ex.VAT

Note: The above costs are for guidance only. Your local authority website will have a full fee schedule for you to download, alternatively an Approved Inspector will provide quotes on a project-by-project basis


Large single storey rear extension with internal reconfiguration and cosmetic upgrade.

Costs overview

The table below gives an overview and estimated price ranges for how much your house extension may cost.

Area (m2)
M2/unit price
GF – New worksBasic rectangle, high-end doors/glazing36.4£2,000 per m2£72,800.00
GF – Major structural openingunit£10,000£10,000
GF – Remodelminor structural alterations, new elec/plumbing, cosmetic upgrade65£750 per m2£48,750.00
FF – Internal works and additionsminor structural alterations, new elec/plumbing, cosmetic upgrade30.3£750 per m2£22,725.00
FF – Cosmetic upgradeFresh paint, new carpet, new rads31.2£250 per m2£7800.00
2F – Bathroom remodel (mid-range)4-piece mid-high spec, existing shellunit£10,000£10,000
2F – Cosmetic UpgradeFresh paint, new carpet, new rads33.5£250 per m2£8,375.00
(ex. VAT)£210,450.00

2-Bed Maisonette – Side-return Extension, Remodel and Refurbishment

This type of project is often misleading when taking a £/m2 value because of the large amount of engineering works needed to create a small amount of additional floor area.

Based on our guidance above, a value of £2,500/m2 should be taken here for the extension works. The additional floor area is approx. 15m2, so the guideline cost of this structural work is £37,500.

Taking a figure of £500/m2 for the refurbishment of the remaining 65m2 = £32,500, plus approx.. £6000 for the major structural opening, we’d suggest budgeting £76,000 +VAT for this side-return and refurbishment (excluding kitchen and bathroom sanitary ware).

If you have a project you’d like to discuss, please Create Your Brief now and we’ll be in touch.


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Ben Richards
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