What is the cost of a two storey extension? Plus, how to budget for your addition

Calculating the cost of a two storey extension is important to ensure a build project runs smoothly and on time.

Although the costs of a single storey extension and that of a double storey extension often don't vary too hugely, there are differences that it is worth bearing in mind. To storey extensions tend to require careful design planning in order to work well with an existing house and are also much more likely than single storey extensions to require planning permission.

Building an extension that is two storeys high will also require you to reconfigure your first floor layout in many cases, all of which can add to costs. Here, we take a look at the cost of a two storey extension and explain how these costs might compare to other types of extensions.

Two storey extension cost

The cost of a two-storey extension will be determined by a number of factors, including size and build quality.

Up until recently, two storey extension costs were:

• If you’re extending on tight a budget, expect to pay £1,650 - £2,100/m² for basic quality
• For a good quality double storey extension, you’ll pay between £1,830 - £2,350/m²
• For a high spec extension, calculate spending between £2,150 - £3000/m²

However, price increases in pretty much every area of the building industry of late have changed things somewhat.

“Over the last year we have seen typical two storey extension costs rise from £1,750/m2 to £2,000/m2 and well over, depending on design," says Freelance Senior Quantity Surveyor Tim Phillips. "This has left some clients rethinking their projects with a 15% rise to soak-up

"People could consider project managing their extension themselves to find cost savings or even having a ‘shell and core’ build so they can complete items themselves if they have the necessary DIY skills or are happy to employ individual trades," continues Tim. 

"Both procurement methods will certainly reduce the build cost and hopefully get their extension back on track.”

Factors affecting two storey extension cost

Although it is possible to give a rough figure for what double storey extensions are likely to cost, there are obviously a few variables that will affect the price you are quoted. These include:

• The size of your extension
• How complex the design is
• The quality you are aiming for
• How much of a hands-on role you are prepared to take on
• Who you get to design your extension
• The amount and type of glazing you specify
• What the new extension is for – bedroom, bathroom, kitchen etc.
• Where in the country you are building

Two storey vs single storey extension costs

You may be surprised to learn that double storey extensions actually don't cost much more than single storey extensions per m2. 

The reason for this is that some of the most expensive elements of the build – the roof and the foundations – will still be required for both and will cost roughly the same regardless of whether you are building one or two storeys. 

“One of the major cost elements to consider in a single or two storey extension is the groundworks, however there is no real difference in structural design, so investigating any savings would reveal them to be extremely minimal," explains Tim Phillips.

The only extra costs involved in building a two storey extension, as opposed to a single storey one, are the walls and new floor joists. 

For this reason, many people see two storey extensions as a more cost-effective way of adding extra space to their homes. 

Additional two storey extension costs

Before setting a final budget for your extension, be aware that there are some costs that are often missed off quotes or financial plans.

On top of the build costs, you’ll need to factor in the following:

• Architects’ fees (around 3-7% of construction costs)
• Project Management (around 3-7% of build cost)
• Planning application fees
• A Lawful Development Certificate, if needed
• Survey costs (between £500-1,500)
• Structural engineer (£500-1,000)
• Building control charges (up to £900 for a 80-100m² extension)
• Party Wall Agreements (£700-1,000 per neighbour if needed)
• Interior fit out costs – kitchens and bathrooms, for instance
• Insurance

It is worth noting that material costs and labour shortages are significantly impacting build cost up and down the country, and are frequently changing. A local architect or Quantity Surveyor should be able to assist in the most up-to-date figures and estimates.