Double-storey extensions: from cost to planning permissions

Double storey extensions

Would you like or need a bigger house to live in – maybe your family is growing – but don’t want to move, as it’s a stressful and expensive process? A beautifully designed two-storey extension that will completely transform your home could be the solution.

A double-storey extension works well with detached or semi-detached homes with a large garden to the rear and also when the original house is wider than it is long, because you can create a more square footprint. And, in reality, it’s often easier with detached homes, as your neighbours are less likely to complain.

To help you decide if a double-storey extension is the right choice for your project, KBB journo Amanda Cochrane is breaking down what you need to know, from benefits and planning requirements to cost…

Double storey extension of a Yorkshire property surrounded by greenery.
This two-storey extension of a Yorkshire property by Lark Architects is home to a warm and contemporary kitchen using SieMatic SLX furniture by interior design studio Grid Thirteen.

The cost

While the cost of a double-storey extension may feel prohibitive (around ÂŁ2000 to ÂŁ2500 per sq m, according to Shehryar Khan of Sheraton Interiors), there are plenty of perks to doubling up your extension.

Not only do you add a significant amount of space, which often equates to a lot of added value for your home, but it is relatively good value for money. This is because the cost of making the foundations is spread between the two floors, according to Resi.

Double storey extension of a grand property and a spacious garden.
A 1930s four-bedroom home, which was previously a mix of different spaces and extensions and had poor garden access, has been transformed with a highly sustainable retrofit wraparound extension, first-floor extension, and loft conversion designed by Delve Architects, with interior design by The Want Room. The centrepiece of the kitchen, created by The Main Company, is a generously sized island with a concrete worktop that offers ample worktop space and serves as a sociable hub.

Planning requirements

This is complicated but, for permitted development, the double-storey extension can be a maximum of three metres deep on both floors. You might be able to add a double-storey extension to the side of your property. However, you will need planning permission because permitted development rights are not applicable here.

“Working with a skilled architect simplifies the planning process and ensures a better understanding of building regulations, materials, thermal performance requirements and structural integrity,” says architect Benjamin Wilkes.

“Their expertise makes navigating these complications much easier and helps bring your vision to life seamlessly. They will also be able to guide you on the 45-degree rule – a common planning guideline to protect neighbours’ access to light, this rule ensures that your extension doesn’t overlook or dominate other properties.”

Benefits

If you are going for an extension project and the sky’s the limit – as well as the budget – then the double-storied version really is a game-changer. It can be a very worthwhile investment and that’s because it can totally transform your home, adding a new sense of space and a unique new flow of rooms.

Two-floor renovation project in Clapham clad in bricks and glass.
In order to make space for a two-storey extension, MW Architects dropped the floor at a lower ground level and excavated a portion of the garden. Photography: French + Tye.

Plus, it allows you to be creative. On the first floor, do you need another bedroom, a home office or a studio with views of the garden? On the ground floor, a gorgeous open-plan kitchen and living space with a playroom and bootroom?

The double-storey can make all of that happen, allowing your once-small home to become your forever home.

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