Average UK Double Storey Extension Cost Per Square Metre 2023
You can expect variances in the cost per square metre of a double extension depending on location. Expect to pay more if you live in and around London.
Type of Extension
Extension Cost Per m2
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Approximate Primary Costs Involved in a Double Storey Extension
There are many costs that you simply cannot avoid when having an extension built. This section looks to cover these key costs and concisely explain each one.
Council Application Costs in 2023
The two main types of application that can be required when having an extension built are a planning application and a building control (building regulations) application.
Planning application cost in 2023
You can typically proceed with having a double storey extension built without planning permission as they normally fall under permitted development. The current cost of planning permission stands at £206 along with a £20 processing fee if you submit your documents through the Planning Portal.
Building under permitted development rights will of course save you the planning fee. If you have an extension built that requires planning permission but you don’t secure it, you will most likely face hefty fines and/or lawsuits.
Building regulation full plans application cost in 2023
Building regulations are a set of guidelines that must be followed to ensure that a building is safe and fit for use by both the residents of the building and those around it.
All extensions require the submission of a building control application and the completion of building regulation drawings. A building control certificate is required upon completion of the extension and certifies that all work done on your extension complies with building regulations. Building regulation drawings for a double-storey extension will typically set you back between £1,000 and £1,200.
If you don’t submit a building control application and get it approved, you will have to pay building control fees and you could be made to take your extension down if it doesn’t meet the necessary standards.
Measured building survey for extension
A measured survey will typically set you back between £500 and £1,500. A topographical survey is required on top of this and will cost between £400 and £800.
Architect fees for double storey extension in 2023
Planning drawings of a property are normally undertaken by an architect. Architect fees are typically 10-15% of the overall cost of the work, and this fee covers both design and supervision of the project. Expect an architect to work between 7 and 21 days depending on the complexity of the project.
If you don’t want an architect, you could have your planning drawings done by a design company or a draftsperson. Expect to pay between £2,000 and £4,000 if you choose that over an architect.
Structural Drawings and Calculations
Structural calculations and drawings are done by a structural engineer and will set you back between £50 and £90 per hour depending on their experience, location, the magnitude of the project, how many visits are required and whether they work independently or as part of a firm.
Ground Soil Investigation
Prior to the commencement of building an extension, there is a requirement for a ground soil investigation. A desktop study will set you back around £600 and a complete site soil investigation around £1,000 depending on the location of your property and complexity of the project (fees exclude VAT).
Party Wall Fees
A party wall is a wall that stands wholly on one owner’s land but is used by two or more property owners to separate their buildings.
If your extension project involves building or excavating close to a party wall, then you will most likely need a party wall agreement. You are legally required to inform your neighbours if you plan to carry out any building work on or near your party wall. The overall cost will depend on whether you require a party wall surveyor. Party wall fees typically fall between £700 and £1,000 per neighbour.
Arboriculture Impact Assessment
If your extension project involves trees that could be affected by the build, you will need an arboriculture impact assessment to determine if the trees can be removed. Prices for this service typically start at between £250 and £400.
A tree preservation order is made by local planning authorities to protect trees and woodlands. It is a criminal offence to remove any tree that has a tree preservation order, and you could be fined up to £20,000 if you do.
Certificate of Lawful Development
Though they aren’t a legal requirement, it’s a good idea to obtain a certificate of lawful development once your extension is completed. This certificate proves to your local authority and future buyers that your project was legal at the time of construction. The council fee for a certificate of lawful development is £103.
Community Infrastructure Levy
The community infrastructure levy is a planning charge set by local authorities on new developments. The charge helps to deliver sufficient infrastructure required to support development in the area in which a development is situated.
If your extension increases the floor area of your property by more than 100 metres squared, you might need to pay a fee under the community infrastructure levy. This fee currently stands at £95 per square metre but doesn’t apply to all local authorities.
Flood Risk Assessment
A flood risk assessment covers flood risks to a development site and will set you back around £250 if required.
Site insurance typically lasts for the duration of an extension project and massively varies depending on location and magnitude of a project. Expect to pay between £500 and £1,500 for a property measuring 140 metres squared.
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Approximate Construction Costs Involved in a Double Storey Extension in 2023
This section looks to investigate the construction costs that you will face when having a double-storey extension built.
Cost For Different Double-Storey Extension Materials
The cost of a double-storey extension massively varies depending on the primary materials used to form the structure. Timber extensions are super popular as they are reliable and cost-effective. Prefabricated extensions are popular as they are built off-site which consequently reduces labour costs. High-end glass extensions are popular as they blend well with both old and new properties, but typically incur a higher cost than timber and prefabricated structures. Conservatories are typically the most cost-effective option when looking to add more space to your home.
The below table examines some of the variances you can expect to find in material costs.
£1,400-£2,000 per m2
£2,000 per m2
£3,000 per m2
Around £15,000 total cost
Cost of Different Roof Types
The types of materials that can be used on extension roofs also present considerable price variances. The below tables provide some detail on roof material options and costs.
Flat Roof Cost per m2
Pitched Roof Cost per m2
You will most likely face higher labour costs for the building of a pitched roof as they typically take longer to build than a flat one. The building of a flat roof tends to present lower labour costs as they are typically simpler to build and consequently take less time. Flat roofs also typically present fewer material costs.
Other roof options include glazed roofs and green roofs. Glazed roofs typically cost around £1,000 per panel and suit side extensions well. Green roofs provide great insulation and are great for plant growth but tend to be an expensive option costing around £100 to £140 per metre squared.
Cost for Installing Kitchen, Bathroom and Utility Rooms
The installation of kitchens, bathrooms and utility rooms present huge price variances depending on the complexity and quality of the build. The below table estimates these variances.
The cost of installing stairs varies massively depending on materials and quality. You could pay as low as £1,000 for a budget option but could be looking at paying as much as £50,000 for the installation of a bespoke staircase.
The installation of doors on your extension will present considerable price variances depending on style, size, and materials used.
Though they are an expensive option to install, bi-fold doors are extremely popular in 2021 and can add up to 20% of value to your home, making them great long-term investments.
The below table provides estimates on the variances in price you can expect when having doors installed.
Bi-Fold External Doors
Wooden Internal Doors
UPVC External Doors
Wooden External Doors
£1,500-£2,500 per linear metre
£50-£500 per door
£150-£500 per door
£100-£500 per door
The installation of windows on an extension are vital as they provide natural light. UPVC windows are the most common type of window found on extensions in 2021. Two large UPVC windows will typically set you back between £200 and £600 depending on quality and mechanical complexity.
Flooring and Finish Costs
Flooring and finish costs vary based on quality of materials, property location and labour costs. The below table looks into these variances.
£17-£70 per m2
£7-£20 per m2
£4-£40 per m2
£10-£60 per m2
Lighting (Without Fitting)
£50-£500 per wall
£25-£75 per m2
£85 per m2
£10-£130 per tub
£60-£100 per m2
A new boiler system will typically set you back between £2,000 and £5,000 depending on your location and the size of your extension.
Labour Costs (Per Hour)
Labour costs vary depending on location and complexity of the work done. Most online quote calculators don’t factor in VAT, so be sure to factor it in when determining your project costs. The standard rate of VAT in 2021 is 20%.
Home extension Builder
£9-£40 per hour
£30-£50 per hour
£25-£120 per hour
£100-£150 per day
Time Spent on Project