How to Build an Annexe or Office in Your Garden
If you need extra accommodation or space for an office, a garden outbuilding could be the perfect solution
Professional advice from: Andrew Guppy of The Classic Barn Company; Abbie Elson of Prime Oak; Angus Eitel of Fiftypointeight Architecture + Interiors; Jakki Cosgrove of JML Garden Rooms
If your annexe is to be used as accommodation, you would usually need to apply for Planning Permission.
“Each council has a slightly different protocol. However, as a general rule, a full planning application takes up to 10 weeks (pre planning applications less),” Andrew Guppy says.
“You’re not necessarily restricted in the same way with heights as you would be if you were building a garden room under Permitted Development [maximum height under these rules is usually 2.5m],” Jakki Cosgrove says. “However, the scale of those parameters sits well in domestic environments.
“Good communication with the planning department is essential,” she adds, “and pre-application guidance is generally available from local councils if you want to establish the principle before advancing too far into the project.”
“Converting an existing building into a liveable space is always an option,” Andrew suggests. “It’s best to start by assessing its structural integrity along with its current insulation requirements, in particular the floor, which will probably need insulating along with the walls.”
“It is possible in certain circumstances to build annexe-type buildings under Permitted Development,” Abbie Elson says. The intended use of your outbuilding, as well as some other factors, will determine whether or not you need to apply for Planning Permission.
“As long as the proposed use is incidental to the enjoyment of the dwelling house, then it would be acceptable,” Angus Eitel says. “In other words, it can’t be ‘primary’ accommodation. A home office, for example, falls within this proposed use.”
Angus lists the following rules that buildings need to comply with for Permitted Development:
- No outbuilding on land forward of a wall forming the principal elevation (ie not at the front of the house or jutting out from the side).
- Outbuildings to be single storey, with a maximum eaves height of 2.5m, and a maximum overall height of 4m for a dual-pitched roof or 3m for any other roof.
- A maximum height of 2.5m in the case of a building, enclosure or container within 2m of a boundary of the curtilage of the dwelling house.
- No verandas, balconies or raised platforms (a platform must not exceed 0.3m in height).
- No more than half the area of land around the “original house” would be covered by additions or other buildings.
- In National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites, the maximum area to be covered by buildings, enclosures, containers and pools more than 20m from the house is limited to 10 sq m.
The first thing to consider is how much space you have and how close the outbuilding will be to other structures. “It’s best practice to leave a minimum of 1m between the new building and surrounding boundaries and structures,” Andrew says.
Angus also highlights the importance of considering the position of trees to avoid damage to roots and to think about the impact on access and parking.
“Ensure the ground is level and cut back any foliage,” Andrew says. “Also check if there are any underground or overhead services, such as power cables.”
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“Foundations are fairly straightforward for ‘light’, single-storey, timber-framed buildings and can be installed using ground screw systems, which reduces the cost,” Angus says. “This will be determined by site access and ground conditions.”
Abbie recommends a traditional strip foundation, which involves filling shallow trenches along the perimeter of the house and load-bearing walls with concrete. “Our design team will create groundwork drawings for a builder to follow,” she says.
“Services and utilities are a key component,” Angus says, pointing to power, heating, water supply and drainage as necessary requirements.
He suggests installing photovoltaic solar panels and a storage battery for a more energy-efficient supply of electricity. He also recommends using super insulation, an airtight construction, and a mechanical ventilation with heat recovery (MVHR) system to reduce heating bills.
If these are installed, Angus says, it’s very possible “to achieve net zero carbon for an outbuilding, and have it ‘off grid’, with no requirement to connect it to the mains electrical supply to the house”.
Don’t forget about broadband, too, Angus adds. “Relying on Wi-Fi may cause problems, so hardwiring a connection from the broadband router will be required,” he says.
More: How to Retrofit Healthy Ventilation into Your Home.
“If a building is erected under Permitted Development rights, there’s freedom with the choice of materials,” Angus says. “If Planning Permission is required, there may be more limitations in terms of materials.”
He says choice will be determined by whether you want to complement the original building or create something different, and suggests you consider the lifespan, cost, embodied energy and maintenance of the materials.
Wood is a popular choice for outbuildings, say our experts. “Timber is a natural, renewable material; it’s quicker to build and more forgiving to changes,” Andrew says. “Above all, it’s aesthetically softer and tends to fit into garden and rural surroundings.”
“Some timber, such as cedar and larch, can be left ‘natural’ to weather, so won’t require repeated maintenance, such as painting,” Angus says. “Kebony and accoya are modified softwoods from sustainable and renewable sources. They have very good durability, acting like hardwoods, and will last up to 80 years untreated.”
Andrew highlights the longevity of seasoned oak and adds, “Timber properties from the 16th and 17th centuries are still standing, providing reassurance they’ll stand the test of time.”
“Both types of roof design have their merits and its very much down to the client’s personal preference, although there may be occasions when one is better suited to the surroundings from a planning perspective,” Jakki says.
“Pitched roofs offer a more traditional style and by using a SIPs [Structurally Insulated Panels] kit, you can create an added feeling of space internally with a vaulted ceiling,” she says. “Flat roof garden rooms are more contemporary, but can sit in harmony with both modern and traditional homes, complementing their style.”
“Green roofs are increasingly popular and good to look at from the windows of the main house. They also have limited maintenance,” Angus says. “Also think about the drainage of the roof. The water shouldn’t go to mains drainage; rather, a small soakaway should be located to allow natural drainage on site.”
He also suggests incorporating an overhang if you’re concerned about overheating in the summer.
Andrew recommends hardwood or aluminium, as these are the most durable options. “I don’t include plastic purely on the basis that it doesn’t lend itself well as a finish against timber,” he says.
“We’d recommend double or triple-glazed windows in aluminium or composite timber frames,” Angus says. “It’s also worth thinking about rooflights, as they can offer a good level of background light.
“We suggest the use of local manufacturers for the supply of windows and doors, with timber that’s sustainably sourced as locally as possible,” he says. “Recycled aluminium lasts incredibly well without rusting or rotting.”
If you’re planning to paint the wood, Abbie suggests using western red cedar, as it’s very stable and easily painted.
A key piece of advice is to hire a professional who can manage the whole process for you, from planning to design and build.
“Ensure your supplier can confirm the garden room complies with the relevant Building Regulations. If they have the facility to carry out applications on your behalf, utilise this service for peace of mind that the proper consents are in place before you start your project,” Jakki says.
Andrew agrees and adds that ‘one contractor accountability’ is among the key attributes a provider can offer to make the project quicker and easier.
Are you planning to build an annexe or office in your garden? Was this advice useful? Share your thoughts in the Comments.