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Lesley Hally is principal architect at LA Hally Architect, an award-winning RIBA Chartered Practice with a fresh and innovative approach to house design.
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An Architect’s Guide to Designing & Building an Annexe – How to Get it Right
Maximising your plot by building an annexe could be the answer to creating more space at home, says Opinder Liddar. This annexe design guide looks at the key elements you need to consider for a successful design – from planning permission to connecting to services
Are you considering building an annexe on your plot? There are plenty of benefits to creating an extra structure alongside your main home. As we’ve watched our children grow, for example, it dawned on us that we needed more space for family members to do their own thing. Initially we thought about moving, but we like where we live and larger properties nearby are outside of our price range. Once we decided to stay, I looked at our property and plot to consider how we might achieve some much-needed extra space. We’re now proud owners of an annexe at the bottom of our garden.
In this annexe design guide, I’m looking at the key factors you need to consider before getting ahead with a project – from planning permission for an annexe, through to the site’s potential, the structure’s impact on your main house, its size and more.
Do you have enough space for an annexe?
Before planning an annexe building, you must ensure you retain enough outdoor space to enjoy fresh air and sunshine. Review the periphery of your plot, and the back in particular – you may have areas that are used infrequently, or not at all, and could be a suitable location for an annexe.
Our home had a garage at the bottom of the garden, which was reached by a narrow track. Beside the garage, we had some vegetable patches and a patch of unruly planting. When I measured and drew this area out, I was surprised to see just how much land was available.
How will the annexe impact the main house?
In terms of where you can build, you can go right up to your boundary, however, by doing so you may be limited to windows on one side only. If that’s the case, use rooflights to balance light levels deeper in the floorplan of the annexe. It’s likely that windows will to look out to the main house, so if the building is relatively close, think about using planting to screen the view or obscured glazing in places.
If you live in an urban, terraced house that has a rear exit, it’s not a good idea to build from boundary to boundary and block your access. At the very least, it will be useful during construction.
Achieved in tandem with a single-storey kitchen extension to the main home, this project by LA Hally Architect involved converting and extending the property’s existing garage to form a light-filled, two-storey annexe. Photo: Richard Chivers
You must consider the orientation of your annexe; make sure its position doesn’t create excess shade (although a little shade will offer a comfortable seating spot in high summer). You should also ensure that the window positioning doesn’t create any overheating issues. If your glazing will be facing the sun throughout the day, you’ll need to consider solar shading and ventilation measures. You need to take note of trees on site, too, and aim to stay at least five metres away so that roots don’t cause havoc under the building in the future.
How do you connect an annexe to services?
Depending on the distance your garden building is from the main house, you’ll need to consider getting services to it. In our case, I laid new ducts between the house and annexe, through which we laid electricity, water and a data cable for internet access.
One of the challenges you may face is drainage. The mains drains are likely to be near the house and probably close to the ground level surface. This was a dilemma with our project, as it was not possible to rely on gravity to create the necessary fall.
I installed a relatively inexpensive mini-pumping station to overcome this. These stations can cater for very long distances and are well worth pursuing rather than a septic tank, which will take up more garden space and must be regularly emptied.
How big should your annexe be?
In theory – to meet permitted development (PD) requirements – your annexe could cover half the area of land around your house, which could be quite considerable. Think about how you would like to use your new space and make a list of your requirements.
In my case, it started with the dream of a full-sized pool table and went on from there. We wanted to accommodate a bed, a shower room and some storage space, which could easily convert to another separate room in the future.
The charming, two-storey oak frame annexe by Carpenter Oak features an air source heat pump, with solar panels on the roof, a Tesla PowerWall battery, and is insulated with mineral wool and wood fibre insulation. Photo: Claire Williams
Think about flexible use, too. Throughout the pandemic, our annexe has been two home offices, a play space, an exercise studio, a home cinema and music room – at times with drums in one room and a clarinet in the other. The space has supported the whole family and could not have been more flexible. Try and preserve height in part of building if you can – I’ve managed to keep one area vaulted and the remainder has a low, flat ceiling.
If built properly, an annexe can last as long as the main house so think about your future needs (futureproofing). Simple measures like wider doors and introducing at least one level threshold are vital for wheelchair access. Should the need arise in the future, we have a space to look after a family member that might need our care.
Case study Striking self build annexe on the Cornish coast
Suzanne Mackey has owned a home in Cornwall for more than 17 years and, in that time, she’s renovated twice. Yet when she applied for planning permission for the second renovation in 2019, Suzanne took a chance on a significant addition at the last minute.
Photo: Simon Burt
“I threw an annexe into the application, thinking we wouldn’t get it,” says Suzanne, who was surprised when the planners approved it. Three years later, the striking contemporary addition provides a private space boasting sea views; the perfect spot to accommodate her family and guests.
The property is located on a country lane, bounded on one side by an area of outstanding natural beauty (AONB). Situated on a hill, the main house enjoys 180° vistas of the North Cornish coast. A secluded 1.5-acre garden stretches down towards the sea, a quarter of which was “dead space,” according to Suzanne. Not overlooked by neighbours, this made it the ideal spot for a new annexe.
Photo: Simon Burt
“I really like modern buildings, but my main home is very traditional, and I wanted something different,” she says. Suzanne gave her sketches to local practice CFD Architects, who were happy to formalise her plans. The charred timber cladding is a key feature of the annexe, which has been treated in the Japanese method known as Shou Sugi Ban. “It was an expensive part of the project, but crucial,” says Suzanne.
| Location | Cornwall |
| Construction method | Masonry with timber cladding |
| Project route | Owner managed alongside main contractor |
| Size | 111m² |
| Project cost | £315,500 (£2,842 per m²) |
| Building work took | 7 months |
Do you need planning permission for an annexe?
If your new building is going to be used as a studio space, a home office or playroom, then you could build within permitted development (PD) rights. If you create a building, which you subsequently want to convert into a self-contained annexe, this might also be considered acceptable without full planning permission.
In this scenario, I suggest you apply for a lawful development certificate. It confirms the annexe falls under PD, or if not, you know you’ll need to apply for formal consent. If, from the outset, you know you’re going to build self-contained residential accommodation, then planning will be required. You’ll typically require planning permission if the eaves are higher than 2.5m or the ridge is higher than 3-4m, too.
In presenting your case, you will need to demonstrate that the proposal will not cause any significant harm or impact to your neighbours’ properties, and include access details and where visitors would park.
This project by Snell David Architects involved converting the site’s existing single-storey listed barn to create ancillary accommodation and a new, light-filled kitchen
In my case I wanted a dual vaulted roof and to build close to one boundary so I decided to go through the planning system. The design had no impact on our neighbours’ amenity, but, crucially, I also showed I could build something bigger through PD.
The scheme was passed without fanfare and hassle so don’t be concerned if what you want to achieve needs a planning application. Just make sure that you can demonstrate a ‘permitted development fallback’ position as part of your argument.
Remember, the annexe is ancillary to your main house, which means that it must be used by your household. Renting or providing short term holiday accommodation comes under commercial use and will need planning permission.
Note, if you’re in a conservation area, your home is a listed building, or in a National Landscape (previously area of outstanding natural beauty) or where the building will be close to a highway or footpath, then your PD rights are significantly restricted and you should seek a planning consultant’s advice.
In summary:
- You’ll require formal planning permission if the eaves are higher than 2.5m, or if the ridge is higher than 4m.
- If the annexe is to be used as full-time living accommodation or if you plan on renting it out, you’ll require formal consent.
What kind of thermal efficiency should I aim for?
I routinely encourage my clients to build sustainable homes, so I wanted to bring the same standards to my own annexe project. I always propose maximising insulation and going for good levels of airtightness to reduce the heat load and running costs.
My annexe design lent itself to timber frame; I achieved U-values of 0.15W/m²Kfor the floor, wall and roof by insulating between the studs, and an internal layer of insulation. I have also fitted an airtight membrane, plus the windows are double glazed and two electric radiators provide heat, if required. Our annexe heats up very quickly and holds the temperature well. Most of the time the radiators are not needed – there is low demand so the running costs reflect this.
ByOthers Studio designed this 30m² timber frame garden building, which sits atop a timber floor structure and ground screw foundations. The stylish project features a glass pivot door and Corten steel cladding for a unique finish, with sheepswool insulation to maintain a comfortable interior throughout the seasons. Photo: Lorenzo Zandri
When external temperatures are very high, the insulation keeps the building cool inside, and we can also open high-level rooflights if it gets too hot, which creates a solar chimney that draws out the heat. If you’re thinking of buying an off-the-shelf solution, look at the thermal performance as well as how easy the building is to construct.
Whilst they may be cheap and quick to build, you may find heating and cooling costs a fortune, resulting in an uncomfortable space that becomes redundant after little use.
With small buildings that will be providing living accommodation, you could install an electric boiler and underfloor heating (UFH), and even solar PV panels if the annexe is facing the right direction.
Who should build and project manage your scheme?
I used a main contractor to get the structure to an almost watertight stage. I then project managed trades (as an architect) and bought materials myself, setting up a number of accounts with builders’ merchants and suppliers. Through the trades, I quickly learnt which items to buy from where. I also carried out some of the work myself, such as the insulation, so I know that all the joints are tight and sealed.
The cost of your annexe project will depend on how long you need the outbuilding for and how much you choose invest in thermal performance and airtightness. But managing the project this way probably reduced my overall costs by at least 30% in comparison to handing the whole job to one contractor.
Building our annexe has meant we can stay living in our house for longer. It provides us with a hub for a range of activities, and a place for friends to stay.
Self build annexe FAQs
Should my annexe match the existing house, or can it look quite different?
We have worked on several annexes and found that it does make sense for the structure to complement the main building. This could be achieved with the exterior materials, shape and form, orientation etc. The annexe doesn’t need to look like a mini version of the main property, or have to be styled to match exactly, but using a palette inspired by the original build will ensure everything marries together well.
Focussing on a key material or texture from the main house, or the surrounding context, is a good way to reference the original but in a more refined way. In terms of the annexe’s context, we often try to tie the structures together with the use of hard and soft landscaping, so they don’t look too separated.
How can I maximise floor space inside an annexe?
It’s important to consider who will be using the space and what for. In terms of the kitchen, look to incorporate a chopping board over the sink to gain extra surface. Building in storage solutions into every available nook is great, too, although this can be expensive. The bathroom should be as compact as possible without compromising comfort. Consider a built-in cistern and storage, and perhaps a wet room with alcoves to avoid restricted cubicles.
We try to maximise every bit of unused space to design in shelves etc, such as high level areas above kitchen units and showers. If rooms are to be more multifunctional, you could consider foldaway furniture (such as wall beds). Underfloor heating, infrared ceiling mounted panels or skirting level heating will avoid using up valuable wall and storage space, too.
Should I incorporate sustainable features into my annexe design?
When designing environmental features into an annexe, the same care and attention is required when creating a larger-scaled building. We try to avoid too much solar gain by considering the position of the building on the site. If this is tricky to avoid completely, it’s sensible to incorporate external overhangs and solar shutters, rather than just relying on internal blinds and curtains. We are finding more and more clients want to incorporate bee bricks and bat boxes, too, to support biodiversity, and have had the chance to add green roofs to some outbuilding projects.
Ben Hancock-Parr is architectural services manager at Carpenter Oak. Carpenter Oak has crafted bespoke structural timber frames across the UK for nearly forty years, using oak, Douglas fir and glulam to create unique homes, extensions, annexes and outbuildings.
Will an annexe add value to my property?
An annexe will usually add significant value to your house. Homes which can allow multi-generational living are becoming highly sought after, such as those with nearby outbuildings for elderly relatives. Naturally, for a future purchaser to value your annexe, it’s essential that it’s well constructed and has all of the required planning or Building Regulations approvals in place. It will also help if the build adds to your house’s overall aesthetic. We find that our beautifully hand-crafted oak buildings are extremely desirable when they come to market.
What should I factor into a budget when building an annexe?
When figuring out how much to spend on a project, you’ll need to factor in the costs for design, planning permission, construction materials and labour, plus any renewable technologies such as solar panels or an air source heat pump. Consider the size and complexity of the annexe, too, as well as the quality of materials and finishes you’re after. Crucially, you also need to allow for VAT, as unlike new homes, this is chargeable at 20%.
Opinder Liddar
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