How to design a single storey extension

Find out how to design a single storey extension that complements your existing property and creates the space you need

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Choosing the design of your single storey extension is the most important decision you will make when planning your project. Your extension's design will dictate not just its appearance, but also how long it might take to build and how much it will cost. So it pays to make sure you are happy with the design of the extension, the materials you use and its cost-effectiveness. Don't forget, too, that your extension's design can even raise – or lower – the value of your property.

For info on planning and costing your extension, read our essential guide.


Related articles: The ultimate guide to extending your home | Planning and costing your single storey extension | How to add a two storey extensionThe free Real Homes extension cost calculator

What do you want from your extension's design?

‘It is good to have a brief for your designer, but this can be loose,’ says Alan. ‘If you want an open-plan layout, a better relationship between house and garden or simply more space, your designer will advise you on how to achieve this, perhaps with options. If you want expensive items, such as a certain brand of kitchen, or bi-fold or large sliding doors, let the designer know so the added cost can be factored into your budget.’

Think about the future, too. ‘Single-storey extensions usually need less substantial and, therefore, cheaper foundations and steelwork/lintels over openings than a two-storey design. However, if you think you may build another storey above your extension one day, it may be worth planning for this expansion from the outset,’ says Alan. ‘If you are thinking of doing this, make it clear to the professionals you hire.’

Here's what to consider in your extension design planning: 

Deciding on a single storey extension's footprint

While usually determined by the space required within the property, the footprint is defined by factors such as the proximity to neighbouring boundaries, the position of existing windows, and its orientation towards light, views and garden area.

Planning constraints will also be a factor, although many smaller single-storey extensions will fall within the criteria for permitted development, and so will not need a planning application. It is essential at this stage to consider the roof design – in particular, the height of the ridge and the impact on existing first-floor windows.

How to choose a roof for a single storey extension

The roof should complement the materials and lines of those of the existing house, bearing in mind that the higher the ceiling in the new extension, the brighter and bigger the space you’ll end up with. However, be careful that the extension's roof doesn't compromise existing first floor windows. 

Or, if a low pitch is your only option – whether due to planning constraints or not – consider how you can maximise the impact of rooflights; for flat roofs, consider roof lanterns, which will make the ceilings look higher from inside than they would if solid.

A smaller, narrower extension can usually be covered by a mono or duo pitched roof, however, the spans that can be achieved are very limited. A very low roof pitch may be an option, but this may not work aesthetically and could cause problems when obtaining planning permission. Intersecting pitched roofs can work, but often a flat roof is the only solution for a larger extension.

In this instance, there are only a few design styles that really work, and the choice is between a contemporary flat-roofed 'box', usually featuring a lot of glass, an orangery-style extension, or a more traditional conservatory. Another option is a parapet roof – this has an area of pitched roof around the eaves, which conceals a flat roof over the extension and gives the appearance of a traditional pitched roof.

Choosing doors and windows for extensions

Doors and windows will have a major impact on the finished look of your room so ensure you get their positions, sizes and shapes, framing materials and sightlines just right. They will also affect how much daylight reaches the room – when, and where. 

There are few constraints on the position of doors and windows, unless they will impact directly on neighbouring boundaries. Their size, style and position should be determined by the extension’s architectural style – this being one of the most important defining features. 

It's important, too, to ensure their style and framing complements not just your interior fittings – such as your kitchen design – but also the style and framing of your home's existing windows. Something as simple as having the exterior frames in the same paint shade as existing window frames can help the new extension blend with the original house. 

Opt for the widest panel possible when you’re choosing bi-fold doors for uninterrupted outlooks to your garden space. ‘The maximum panel width for this door style is 1.2 metres, providing a large expanse of glass and maximising the view outside,’ explains Neil Ginger, chief executive officer at Origin.

‘There are a range of different options available for bi-fold doors in a single storey conversion, from two-door models through to large eight-door configurations, set-ups for bays and 90-degree corner sections,’ continues Neil Ginger. ‘The price of doors varies but, as a guide, a bespoke, aluminium design would cost from £1,500 per door leaf.

‘The space allowance for the doors to open outside depends entirely on their width, which can be as narrow as 40cm, protruding less than half a metre outwards. You will need to allow just over a metre of space for doors with a width of 1.2 metres.'

Read our guide to choosing windows sympathetic to period homes or go contemporary with metal-framed windows.

The opportunity to bring light in from above the single-storey extension, such as with rooflights, should not be ignored.

Maximise the opening from the existing house

The wider and taller the opening that links the existing house and new extension, the more they will feel like a single room.

All new openings will need to be spanned by joists, usually steel, to support the walls and floor above. The joist size and its supports should be calculated by a structural engineer (you can find one via the Institute of Structural Engineers at The smaller and less visible these elements, the more seamless the flow between old and new. In most instances it is possible to conceal the joist within the ceiling void, especially if you’re removing only a non-load-bearing partition wall.

Create a continuous ceiling level

The ceiling height between old and new spaces should, ideally, be the same. If they’re different, however, the higher ceiling can often be brought down by adding new battens and plasterboarding over the top.

There is no minimum ceiling height under the Building Regulations, other than above staircases, but 2.3-2.4m is standard. If this is not a practical solution, then it is best to have a smaller opening with a boxed bulkhead to conceal the step-in ceiling levels.

Make sure the flooring is laid at the same elevation

When setting out floor levels for an extension, it is important to work backwards from the finished floor level in the existing property to ensure they will be identical once they’re linked.

When you’re remodelling, rather than extending, any differences in floor level can be overcome by building up – often using a quick-setting silicone floor screed. If the same level can’t be easily achieved, it is best to create a full step, (H)19-22cm, rather than a small difference that could end up being a trip hazard for children.

Use finishes that match those in the existing house

Old and new spaces can be linked seamlessly by using the same flooring material throughout. This principle also applies to the same architectural detailing such as windows, doors, skirting, architraves and coving; and the same décor, including colour schemes, flooring, curtains and furniture.

Picking exterior materials for your extension

Exterior materials should be chosen to either match those of the original house – think reclaimed bricks to make your extension look like it’s always been there – or picked to contrast with the original house, such as cladding and render or even glass.

Choosing contrasting materials is sometimes more acceptable to planning departments and an architect can help with this, but make sure the contrast is complementary and not grating visually.

Inside, try to think of your furniture choices for the interior and exterior together. ‘Choosing matching finishes, colours and textures for outdoor and indoor furniture is a great way to keep a coherent look that flows from within the home and out into the garden or patio,’ says John Sims Hilditch, co-founder of Neptune.

Single storey extensions to inspire yours

Use these single storey extensions to inspire the design of yours.


Traditional single storey extension

Adding a single storey side and rear extension to a detached house to update a dated 1970s kitchen into a spacious kitchen-diner.

The design: A side and rear extension to a detached house





‘The owners asked us to turn the small existing kitchen area into a space where they could cook and still feel connected to family life. With this in mind, we created a discreet side and rear extension that mimics the proportions of the original property and measures 14 metres in length. It forms an open-plan space that spans the front to the rear of the house, with a breakfast room to the front and a kitchen at the back.

‘There is a bespoke roof lantern to bring natural light into the extension, plus glazed double doors to the rear and side now connect the internal area to the tiered garden, improving access to the outside space.

‘As well as installing rooflights, we used reflective materials to maximise the daylight, which is important to the feel of the extended ground-floor space. While most of the ground floor is now open-plan, a separate central area, which receives the least daylight, has been used to create a utility room.’

Simon Smith


Garden room extension

Replacing a conservatory with a full width rear extension to a detached house in a Conservation Area.

The design: A full-width extension to a detached house





‘Although the extension was quite large for a Conservation Area, and beyond the homeowner’s permitted development rights, the planning authority took the view that building a sympathetic design with the same footprint as the existing conservatory would be an improvement.’

‘Internally, the feature glazed gable, vaulted ceiling and rooflights help to provide a light, contemporary and luxurious feel in the new space, and shed ample light into the rooms behind. The extension was painted to merge old with new, and the existing natural slate roof was replicated on the extension to achieve a simple and consistent look.’

-Paul Day

Picking the best materials for your extension


Matching existing and new materials so they join seamlessly is often desirable, but is very difficult to get right. Choosing contrasting but complementary materials can work equally well, and is often a much easier alternative.


Single storey kitchen-diner extension

Adding a kitchen-diner extension with a level threshold leading out to the garden to a detached house.

The design: A single-storey kitchen-diner extension to a detached house





‘The owner wanted a light, airy open-plan kitchen-diner and family living space. We designed an orangery to suit both the original style of the the L-shaped house and the previous extension. The new space has bi-fold doors to bring the outside in, plus a rooflight to add height and light to the kitchen area. The result is a highly functional, modern living space, perfect for a growing family to live and entertain in.’

-Paul Schofield


Contemporary single storey extension

Adding a rear and side-return extension to a terraced house to create a kitchen-diner with a seamless link to the outdoor area via folding-sliding doors.

The design: A single-storey rear and side return kitchen-diner extension to a terraced house



riach architects contemporary single storey extension


‘We were asked by the owners to create a light-filled rear extension that would result in a family-sized kitchen, dining and garden room. The existing groundfl oor walls at the rear of the house were removed so that the first-floor accommodation appears to float above the new open-plan room, giving the period property a new contemporary facelift. Large, glazed folding-sliding doors link the new room to the garden for a practical, family-friendly space, with integral roof windows that serve to create a spacious and bright area.’

-Corin Rae

Open-plan extension

This 1920s mid-terrace house has had a single storey rear extension and a loft extension added to house a large, open-plan kitchen, dining area and living space for the whole family

The design:A single-storey rear infill and loft extension to a terraced house





‘This large 1920s mid-terrace property was in need of a complete overhaul. It hadn’t been updated during the last 50 years, but was in a good structural state. All the services were in need of an upgrade, and the owners wanted to reconfigure each floor to optimise the flow between different areas and maximise space.’

‘The vision for the kitchen, dining and family living space at the rear of the house was achieved with an infill single-storey extension leading on to the garden, which created a safe place where the children could play and the adults entertain. The flat, modern rooflights bring natural light deep into the room.’

‘The aim was for the extension to flow seamlessly into the outdoor space. This was achieved with a threshold that sits flush with the patio level, and bi-fold doors that stack neatly to one side. Mood lighting highlights the tiered garden to create a stunning view from inside the house.’

-Simon Smith

Small scale extension

Adding a single storey extension to a semi-detached house and converting the loft to create an extra bedroom. The extension, plus a reconfigured ground floor layout, has made room for a family kitchen-dining area.

The design: A single-storey rear and loft extension to a semi-detached house





‘This house was in need of a complete refurbishment, so the brief was to extend and reconfigure the existing ground floor to create a large family space, as well as adding an extra bedroom upstairs.’

‘Most of the additional space that the homeowner wanted downstairs was found by remodelling the existing rooms, and therefore the size of the extension itself was fairly small. Adding two sets of doors has enabled direct access to the garden from the extended dining and living areas.’

-Jude Tugman

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