Craft Bothy - Bothy Stores; Photo credit - Bothy Stores
Compared to other types of home extension, garden rooms are a cost-effective way of creating additional floor area in a property. They offer a private space away from the hub of the main building, and are perfectly suited to activities that benefit from a degree of separation – such as a home office or music room.
As garden rooms are effectively self-contained new-build projects on a small scale, they can be built very quickly to incredibly high building standards.
Hiring an architect to design your garden room will elevate it above the ordinary and make it something special.
Garden rooms can be used for a wide variety of activities including:
The use of the building can determine whether you require planning permission and building regulations approval. More on this below.
You can only build a garden room without planning permission if your property has permitted development rights and the design of your proposals meets the latest technical guidance. Otherwise, you will require full planning permission. Find out more in our article - Can I build a garden room without planning permission?
Most houses have permitted development rights.
Houses in a 'designated area' – Conservation Area, National Park, Area of Outstanding Natural Beauty, World Heritage Site or the Broads – may have their rights restricted.
Properties that do not enjoy permitted development rights include:
Any garden room that contains a kitchen, bathroom or bedroom will require planning permission.
If you intend to use your garden room as self-contained living accommodation, such as a guest annex or AirB&B, permitted development does not apply and you will require planning permission. More on this below.
It is important to check local planning policy before you start your project. Hiring an architect will help you through this process.
Provided your property has permitted development rights, the permitted development rules for outbuildings, which garden rooms fall under, are broadly as follows.
Size
Positioning
External
Use
In addition to the above, if your garden room changes the character of your property, for example, if you use it to run a business which results in increased traffic, noise or smells, the council may deem your home has become a business premises and request you seek planning permission for its continued use.
While permitted development simplifies the planning process and gives greater assurances of what is possible from the outset, it does not always offer the best planning route.
In some instances, it can be more beneficial to seek planning permission; for example, if there is local precedent for a building, or building use, that would not comply with permitted development rules.
Further, if you have a large garden with separate rear access, and have the potential to build self-contained guest accommodation, full planning permission will maximise your garden room’s potential.
It always pays to carry out a planning search and to appraise local planning policies before deciding on permitted development.
Watson House Annexe - Strom Architects -Photo credit - Richard Chivers
Separate to planning is building regulation approval.
Building regulations set the minimum technical standard for the design, construction and alteration of buildings, ensuring they are safe for habitation.
Some building works require formal notification to the council or a private building control company, while other works can be self-certified by a registered tradesperson.
You do not need to formally apply for building regulations approval if your garden room:
Electrical, plumbing and heating works will need to be carried out by a registered competent tradesperson who will self-certify the works and issue certification showing that the installation is safe and meets building regulations.
If your proposals include new sewer connections, you will need to apply to building control, and possibly your local water company, for approval.
It is important that new foundations and groundworks are properly designed and installed. Even if your garden room does not require building regulations approval, it is advisable to seek advice from a structural engineer or garden room specialist to ensure the new building is properly supported.
If building regulations approval is applicable, you would normally apply once the planning stage of your project is complete.
As with other building projects, garden room costs are determined by factors such as size, location and specification.
Professional fees and installation costs will also have an impact on budget. Taking a DIY approach is likely to be more cost effective than a complete turnkey package from a specialist supply company.
Below are some budget figures to give you a rough guide.
Expect to pay about £15,000 to £20,000 plus VAT for a small modular garden room built to a good specification, enabling all-year use.
A similarly appointed medium sized garden room will cost from about £25,000.
Adding a WC will cost about £3,000, while a compact shower room will add about £5,000.
You can install a small kitchenette for as little as £2,000.
There are online companies selling garden rooms from as little as £5,000 but they are often poorly insulated and built from low-quality materials. These products are unlikely to last long and are expensive to run during the colder months.
If you are building on a sloping site, or require specialist foundations, you will need to allow for additional costs.
Items such as built-in furniture, planted roofs, underfloor heating and decking are optional extras.
If you want a bespoke design, you will need to account for professional fees and builder costs. Architect fees are likely to start from about £1,500 plus VAT for planning and technical drawings. A structural engineer will charge around £500 plus VAT to design the foundations and groundworks, if required. Build costs start from around £2,500 plus VAT per square metre.
If you require planning permission, a householder application costs £206 in England, £300 in Scotland (£225 if in a conservation area) and £190 in Wales.
Lawful Development Certificate applications cost £103 in England, £300 in Scotland and £85 in Wales.
The main items that affect garden room costs are:
All garden buildings require a firm base to sit on. The extent of the groundworks depends on the size and weight of your garden room.
If you are installing a modular garden building, your supplier will advise on what is required. Many modular building companies have their own preferred systems to suit their designs. It is important you check whether the price of the foundations is included in the quote.
If your site is flat and ground conditions are favourable, a small, lightweight garden building can sit on a levelling base, often consisting of a recycled plastic grid system filled with loose stone.
For most types of garden room, the best option is to install pile foundations. Holes are excavated at the corners of the building and at regular interval spacings across the base to the required depth – usually about a metre – and filled with concrete. Adjustable steel bracket connections are then set into the concrete and a suspended timber floor is fixed on top.
This foundation system causes minimal disruption and can be easily covered if you decide to move the building in the future.
There are also proprietary foundations systems for garden buildings which can speed up site operations.
Companies such as Swift and EasyPAD offer systems that function in a similar way to traditional pile foundations but consist of interlocking pre-cast concrete plinths and heavy gauge steel brackets for levelling.
UK Helix do a concrete-free system that comprises of steel screw piles.
It is important to consult with a structural engineer or specialist garden room company to understand which system is most appropriate for your site before starting work.
As a minimum you will require an electrical supply, so your garden room is a usable space.
An armoured cable is normally required to connect the garden room to the main supply in the house. To avoid any unsightly cables, the best approach is to bury the cable underground. This involves digging a trench, normally about half a metre deep, covering with hazard tape to warn anyone in the future that cables have been laid, and then filling with stone and then earth.
To complete the installation, you will need to install a new consumer unit in the garden room and an earth rod in the ground close to the garden building.
If you plan to install a kitchenette, WC or shower room, you will need a mains water supply and drainage connections for waste.
The extent of the work, and therefore cost, will depend on existing site conditions. If you have a water supply and sewer connections nearby, and the site is level, odds are the work will be relatively straightforward and costs will be kept to a minimum.
If existing connections are a good distance away, you will need to dig trenches in the ground and install new pipework and drainage runs which can be disruptive and costly. If the site slopes and your garden room is below existing drainage levels, you may need to install a pump or septic tank, adding further costs.
Hot water can be generated through an electric hot water cylinder and solar hot water panels, provided you have the roof area.
When budgeting for your garden room, it’s important to carry out a site survey in the first instance so you can budget for all potential costs.
Achieving a warm, comfortable environment is essential if you want to use your garden room all year round.
Building to high thermal and airtight standards will ensure you keep heating usage and costs down to a minimum.
A well-designed garden room will require very little heat. A portable fan heater with a timer and thermostatic controls will be perfectly adequate.
The Dyson Pure Hot + Cool TM HP00 Purifying Fan or more affordable Duux Three sixty 2 Smart Fan and Heater offer good aesthetics and performance.
For larger spaces, electric convection heaters, electric radiators or oil-filled radiators with timers and thermostats will provide adequate thermal comfort.
If you have limited wall or floor space, or you want to achieve an uncluttered look, consider electric underfloor heating.
As an alternative to electric options, consider a log-burning stove for high heat output and visual warmth. Rais’s Q-Tee range offers high-quality stylish wood burners at various sizes, starting with the 57cm tall stove which is suitable for small spaces.
Wood burning stoves should be installed by a HETAS registered installer to meet building regulations. You will need to build a non-combustible hearth to meet regulations and preferably an are a nearby to store logs.
Garden rooms are mainly built either using bespoke or modular construction methods. Both have their pros and cons. The route you choose will depend on your personal requirements, site constraints and budget.
Modular garden rooms are usually supplied as an assembly of parts designed and installed by the supplier. There are some suppliers which offer systems that can be assembled on a DIY basis.
Modular companies typically supply a range of designs and sizes and offer customisation in the form of optional extras and add-ons, including cladding options, planted roofs, underfloor heating, kitchens, bathrooms, built-in furniture and specification upgrades.
Design options are more limited than a bespoke route, but there are benefits.
The advantages of choosing a modular garden room include:
The disadvantages include:
More: Five architect modular garden room companies
Building a bespoke garden room will ensure your design is tailored exactly to your requirements without any compromises.
The benefits of bespoke garden rooms are:
The disadvantages include:
More: Twenty inspirational architect designed garden rooms
The build time will depend on the type of garden room (modular or bespoke), the size and complexity of the design, council approvals (if required), consultant availability, company lead-in times and site access.
The timelines below will give you a rough guide.
Modular garden room companies typically require a lead in of 8-12 weeks for off-site manufacture from confirmation of order, and 5-10 days for site installation, excluding site preparation and new drainage and utility connections.
Some modular garden rooms are built entirely in a factory, delivered on a lorry and craned into position in a matter of hours!
If you require planning permission, you will need to allow 8 weeks for your application to be decided.
If building regulations approval is applicable, it’s advisable to factor in 3-4 weeks for the design to be signed off. Full approval is awarded after the building works are complete and a building inspector will visit site at key stages of the build to ensure works are built correctly.
If you are making new drainage connections or need to upgrade existing utilities, you may need consent from your local water company and utilities company before starting work.
If you are building a bespoke garden room, you will also need to factor in a period to appoint a design team and complete the design process. If you have a clear architectural brief, allow about 4-6 weeks. The duration of the design programme will very much depend on consultant availability and your ability to provide clear instruction.
Once the building is designed and a price has been agreed with a contractor, a small to medium sized bespoke garden building is likely to take about 4-8 weeks to build. Larger buildings may take longer.
Heavy building finishes that require larger foundations and wet trades, such as brick and stonework, are often best avoided - especially if your building is designed to be lightweight to minimise disruption and speed up delivery.
Timber cladding is popular choice as it works well with modular and timber frame building systems.
Consider a durable softwood such as larch or Douglas fir. Both are grown sustainably in the UK and in neighbouring northern European countries.
Cedar cladding is also widely available in the UK and offers excellent protection from the elements. Most of the UK’s supply arrives from North America and has a slightly higher carbon footprint.
If your budget permits, consider a modified timber such as Accoya. These types of timber are made from chemically treated sustainable softwoods and offer excellent stability and toughness.
Unfinished timber cladding will weather to a silver-grey colour; however, depending on location, aspect and design, it can weather unevenly which is not always aesthetically desirable. For a consistent weathered finish, apply SiOO:X clear coating or similar. It takes around 10-16 weeks to create an even appearance.
SiOO:X Light Grey or Mid Grey will create a consistent weathered look from the outset.
For a dark, stained finish, consider a natural black pine tar coating which provides excellent protection from rot and mould.
Or consider Shou Sugi Ban, a traditional Japanese technique where timber planks are charred to create a blackened durable finish. This works particularly well on cedar.
Other cladding options include timber shingles, standing seam sheet metal (such as zinc) and fibre cement board. Cement board is low combustible and is a good option if you are building close to a boundary and need to meet building regulations.
Finally, exterior grade plywood treated with a protective stain finish can offer something different – see here for inspiration.
Flat roofs are a common choice as they are well suited to a contemporary design aesthetic and help reduce the profile of the building – a requirement if you’re building close to a boundary under permitted development.
They are best covered in a rubber EDPM which can be laid in whole sheets to minimise joints and the risk of water penetration. EPDM roofs typically have a lifespan of over 50 years, much longer than felt and other flat roof coverings.
Garden room roofs are often highly visible from upper floor windows and installing a sedum blanket roof will offer an attractive outlook. As well as being good to look at, sedum roofs increase thermal performance, minimise rainwater runoff and provide a habitat for wildlife.
Pitched roofs can add visual interest to a garden room and create more headroom. Rather than cladding the walls and roof in contrasting materials, which is commonplace in traditional housing, it is often more desirable to create visual continuity and finish them in similar materials. This can help the building ‘sit’ into the garden landscape.
For example, the entire building could be clad in timber, or pair a dark powder coated zinc roof with dark stained timber clad walls.
Garden room interiors do not need to be confined to painted plasterwork. Modular and timber frame building systems lend themselves to alternative finishes such as timber panelling.
Plywood panelling will create a warm, inviting interior space and can be simply finished with clear wax oil – a considerable time saving compared to labour intensive plastering. Many modular building companies run by architects offer plywood interiors as standard.
High-grade birch plywood will ensure a premium finish. Spruce plywood, with its characterful grain and knots, is more cost effective but still attractive.
Plywood can be simply finished with a clear hard wax oil to bring out the natural grain, or stained or painted different colours to create the interior mood you want.
Install plywood pegboard for storage shelves and hooks and to add visual interest. This can be supplied pre-made online, or a CNC workshop can make it for you.
As an alternative to plywood, consider square edge slatted timber sections or painted softwood tongue and groove boards for a more characterful aesthetic.
If your garden room has a suspended timber floor, which is commonplace, engineered timber flooring or plywood is a popular choice.
For a highly sustainable alternative, consider cork flooring. As well as adding character to a space, it provides insulating properties and feels warm underfoot.
More:
An architect’s guide to using plywood in your home
An architect's guide to cork flooring
About the author: Aron Coates is an architect with over two decades of experience, encompassing projects ranging from the restoration of historic buildings to the design of contemporary homes.
The information provided in this article is intended for general guidance and educational purposes only. At Designs in Detail, we advise you appoint a skilled residential architect to provide specific expert advice for your project.
