Two storey rear extension by Grey Griffiths Architects. Photo by Adam Scott.
A two-storey extension can significantly enhance your living space while preserving your garden. They are also a great way of distributing space more evenly across your home, preventing a bottom-heavy layout. This guide outlines the costs involved to help you make informed decisions.
This guide provides a general overview of the costs involved in building a two-storey extension. Building projects are complex, with many factors influencing the final cost. Therefore, online guidance like this should be seen as a starting point for budgeting before consulting a professional.
An experienced residential architect can offer initial cost advice in the early stages of a project, but accurate pricing is only possible once detailed plans are prepared. These allow builders to fully understand the scope of the project and provide precise quotes.
Timber-clad single and double storey extension by Proctor and Shaw Architects. Photo by Jim Stephenson.
As a general rule, a two-storey extension will cost around 50% more than a single-storey extension. This factors in the savings you make on groundworks – i.e., more efficient use of foundations and roofing.
With this in mind, a common method of calculating budget costs, before any plans have been drawn, is to apply a square metre cost rate across the ground floor area and then add a further 50% for the additional storey above. Note that this will give you a figure for the general build costs which covers the shell of the building, utilities, and general finishes. If your extension includes a kitchen, bathroom, or built-in joinery, they will need to be accounted for separately. Landscape works are also an additional cost.
Once you have estimated the rough build costs, you can then work out professional fees and VAT, which can be calculated as percentages.
General build costs – so excluding kitchens and bathrooms as noted previously – for a mid-range extension is about £2,000 per square metre in most parts of the UK, increasing to around £2,600 per square metre in London.
In addition to general build costs, you will also need to budget for interior finishes and fittings. Below are rough costs to supply and install (labour plus materials) to a good quality, mid-range specification:
Victorian style brick extension by Sam Tisdall Architects. Photo by Killian O'Sullivan.
To accommodate the increase in living space, you may also need to upgrade your existing heating and electrical systems. If you’re connecting the levels within the extension, you’ll need to factor in the cost of a new staircase. Finally, the costs outlined here are for the extension only. You will need to budget additionally for any renovation works to the existing building.
In addition to build costs, you’ll need to budget for professional fees and legal approvals. As a rough guide, this may include:
Note that professional fees are time-based and affected by things like the complexity of the design, level of involvement, and status of the site (listed building, conservation area etc.). You can reduce fees by taking on aspects of the project yourself or passing on some responsibilities to the builder (e.g., project management) if willing.
READ: How Much do Architects Cost in the UK?
Renovation and extension projects generally incur the standard rate of VAT of 20% (November 2024). However, certain projects and types of building work may qualify for tax relief. For example, works to homes that have been empty for more than 2 years qualify for a reduced rate of 5%. Sustainable heating systems, such as heat pumps, are exempt from VAT.
READ: VAT on Home Renovations: Your Guide to Standard, 5% and Zero Rates
Two storey extension interior by Oliver Leech Architects. Photo by Jim Stephenson.
General factors such as the size of the extension and specification of finishes and fittings have an obvious key bearing on costs. Other factors include:
Below is a budget cost example for a simply designed two-storey extension measuring 5 metres wide by 3 metres deep inclusive of a kitchen and new bathroom, underfloor heating at ground floor and radiators at first floor:
Total Build Costs: £91,500
VAT @ 20%: £18,300
Total Build Costs Inclusive of VAT: £109,800
Total fees: £13,750
VAT @ 20%: £2,746
Total Fees Inclusive of VAT: £16,496
Grand Total: £126,296
Any works to the existing building will need to be added to this figure.
A compact, fully glazed two storey extension by EBBA Architects. Photo by Rikard Kahn.
Keeping a handle on costs can be difficult as there are so many factors to consider and balance. If you’re working to a tight budget, below are some tips for controlling costs without compromising design quality:
An experienced residential architect will be able to advise on the items that most affect the cost for your extension. They will offer cost-saving advice and suggest where to focus your budget to maximise returns. Architects have a detailed understanding of the entire design and build process and will be able to head off any potential problems before they occur, avoiding unexpected costs.
An architect will help you put together a shortlist of builders, appraise tenders and agree on a contract with your preferred builder for the building work. Then they can monitor and manage the work on site and certify payments to ensure it is delivered on time and on budget.
READ: How to Find and Hire a Builder for a House Renovation or Extension
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, consult a skilled residential architect featured on Designs in Detail.
About the Author
Aron Coates is an architect with over two decades of experience, working on projects ranging from historic building restoration to contemporary home design.
