Two-storey extensions — the expert guide to planning and designing the ideal double-storey addition
Tempted by a two-storey extension? Discover the design options, the benefits, and the rules on expanding the ground and first floor of your home
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Building a two-storey extension is a project that could transform your home. Adding living space downstairs and bedroom space upstairs can save you having to leave a location you love to gain the square meterage you need.
But before you get started on your extension ideas, it’s vital to understand the challenges as well as the possibilities and benefits of building an extension on two storeys, what permissions are required, and how much it’s likely to cost.
Article continues belowIs a two-storey extension a good idea?
Similarly to a single-storey addition, a two-storey extension can make a huge difference to a home. ‘The extra storey provides double the floor space, and allows for far more flexibility in room arrangement,’ says Pereen d’Avoine, founder of architects Russian For Fish. ‘Extensions are often added when families are in flux, if kids are growing up and need their own space, for another baby on the way, even sometimes if older generations are moving in.’
So if you're starting to see signs you should extend your home, it's a good time to start researching your options.
Third-generation architect Pereen d'Avoine founded Russian For Fish. Working alongside her co-director Nilesh Shah (also pictured), the practice offers a full range of architectural and interior design services including new-builds, ceiling-to-gable renovations, extensions and most recently working on double-storey property additions. Based in East London, they work across the capital and throughout the south east.
Bear in mind that a two-storey extension brings design challenges. ‘Absolutely any extension is going to change the look of your house,’ says architect Melanie Clear, founder of Clear Architects. ‘A double storey can also affect the roof so it very much needs to be considered as part of the overall design.’
A two-storey extension also creates issues in respect of the internal layout. ‘The placement of the staircase is key to creating the best layout to both the ground and first floor,’ explains Melanie. ‘It may mean losing a bedroom in the existing property but creating one or two new ones in the extension. The flow of circulation is key and if you do not consider that you may not gain as much new space as you hoped within the extension.’
Melanie is the founder of Clear Architects. She has worked on numerous projects that require re-configuration and this sometimes means the need for a two-storey extension to maximise room space. She always looks at the entire space and designs any extension as part of the overall flow rather than just an add-on.
Do I need planning permission for a double-storey extension?
You won’t necessarily need planning permission for a two-storey extension, but it will depend on the size of extension you want to build. 'Two storey extensions can fall under Permitted Development rights, but the limits are quite strict,' says Ryan Wenham, Chartered Architect and Registered Interior Designer at Larsen Architecture.
'In most cases, a two-storey rear extension must not extend more than three metres beyond the original rear wall of the house, must be at least seven metres from the rear boundary and cannot be higher than the existing roof. Side extensions cannot be more than half of the width of the original house. The roof pitch should match the existing house where possible, and materials should be similar in appearance. Anything beyond the principal elevation(s) is not allowed under permitted development,' he says.
'Planning permission is usually required if you exceed these size limits, live in a conservation area, own a flat or maisonette, or if previous extensions have already used up your allowance. The safest way to double check is to review your local authority’s planning guidance and speak to a planning officer or architect before finalising drawings. Applying for a Lawful Development Certificate is also a sensible step, as it gives you formal written confirmation that your project is lawful before work begins.'
If any of the work impacts a wall you share with a neighbour, then you may need a party wall agreement. It's illegal to ignore party wall legislation.
How much does a two-storey extension cost?
The cost of a two-storey extension can vary widely, according to Rachel Moors, managing director of home transformation experts Adamsons. ‘However, as a general guide, UK homeowners can expect the following per square metre,’ she says. ‘Basic specification: £1,850 per square metre; good specification: £2,200 per square metre; premium specification: £2,600 to £3,500 per square metre.’
These two-storey extension costs cover the structure and basic features which vary according to the specification level, she says. Beyond the structure, budget for costs such as an architect, structural engineer, and fit out for a kitchen, bathroom or bedroom.
You might be tempted to act as your own extension project manager to keep costs as low as possible, but if you do opt for this, you need to make sure you have the necessary skills to successfully complete the task, otherwise it could be a very expensive endeavor.
Rachel Moors is managing director of home transformation experts Adamsons, first established in 1984 by her parents Colin and Susan Moors. Adamsons is a second-generation family business, which has completed hundreds of two-storey extensions across Greater Manchester and Cheshire.
Why is a double-storey extension more cost-effective than a single-storey addition?
When it comes to weighing up a single vs two-storey extension, while it’s true that the final bill for a two-storey extension will be higher than that for a single-storey version, it is more cost effective.
‘A two-storey extension typically offers better value per square metre because foundational and roofing costs are shared across two levels,’ explains Rachel Moors. ‘Unlike adding a second storey later (which requires roof removal and rebuild), building both storeys together means you only pay for one roof structure.’
FAQs
What type of two-storey extension could I build?
While PD requires a rear two-storey extension, with planning permission other types are possible, including side, side return and front. Architect Theo Petrides, director of A21architects, explains each type.
‘Our experience shows rear two-storey extensions usually present the best opportunities for our clients,’ he says. ‘For family households, a two-storey rear extension will often be used to create a convenient, combined kitchen-dining room that opens out onto the garden. At first-floor level, an additional bedroom or en-suite bathroom will usually be added.’
‘Side two-storey extensions are generally small in nature as these are used to infill left-over space; however, these should not be overlooked,’ says Theo. ‘If side two-storey extensions are combined with careful internal reconfiguration of the house, these can be the most cost-effective ways of improving your home, adding bedrooms or reconfiguring to a more open-plan contemporary living layout.
‘Side return two-storey extensions will give you a substantial amount of additional space,’ he says. ‘This type of extension will usually only be applicable to semi-detached or detached houses and cottages where there is enough left-over space and distance to your neighbour’s house to do this. This type of extension will be used to add additional bedrooms at first floor and expand the living space on the ground floor.
‘For detached homes or cottages with a front drive or front garden, front two-storey extensions are an opportunity to completely transform the appearance of your home,’ he says. ‘When working on this type of scheme, A21Architects often looks at increasing the size of the entrance lobby to create a more impressive formal reception space, incorporating double-height space, an open staircase, and WC. This type of extension will also provide ample opportunity to reconfigure the house internally.’
Theo Petrides is a RIBA chartered architect and director of A21architects. Theo's approach in home design and extension architecture is rooted in a deep engagement with clients to understand their unique needs and aspirations. His extensive experience working in and around London, where space is at a premium, informs his belief that it is almost always worth extending with the added benefit of providing opportunity for vastly improved internal layouts.
Design considerations for a two-storey extension
A successful two-storey extension needs careful design. ‘With front extensions or if you are located in a conservation area, the local authority will often require doors and windows to be in keeping, that is, the same as others in the local area, limiting your options,’ says Theo Petrides. ‘However, if you do want something different, an architect can help you justify this to the local authority.
‘Where planning permission is less of an issue, we will often go for contemporary windows that maximise natural light using a slim aluminium or steel frame,’ he says. ‘For historic or more traditional properties, we will closely study the adjacent historic construction methods and propose tried and tested traditional construction methods.
A two-storey extension could give you an extra bedroom to accommodate a growing family or to act as a guest room for friends and family
‘For rear two-storey extensions, we advise maximising the amount of light, especially in urban terraced house extensions. This means full-height sliding-folding doors for most situations, though on larger openings we find simple large panel glass sliding doors function better and last longer.’
Externally, even when not limited by the PD rules, the surroundings of a two-storey extension matter. ‘Proposals will need to relate to their surroundings, whether that’s in terms of the proportions or sometimes simply complementary colours to allow a radical departure on form,’ says Theo. ‘For rear extensions, the role of the garden will really make a difference to how the new extension integrates with the garden landscaping. At first-floor level, extensions will usually be more visible, so will almost always use a materiality that reflects the adjacent buildings and blends into its surroundings.’
