If you have large garden and need additional space for somewhere to work, or a home gym, even someone for an occasional guest, then a garden room might be the best option.

We have done quite a few garden rooms over the years, each one a little bit of architecture. This post tries to give an overview of the essential considerations and restraints you may face so you can be informed and make the right decisions.

Cost

So they vary! Off the shelf prefabricated garden rooms can start at around £1000/m2, whereas a bespoke on site brick building might cost as much as £4000/m2. Lots of things affect the price, such as; ground conditions, material, distance from boundary, height, amount of glazing and level of finish. Some of these decisions may be a choice and sometimes a necesaaity. So what are the main things to consider that might impact the cost:

Planning

You may need planning permsion to build your garden room or you may be able to do it under permitted development. See this post for some inspiration of garden rooms that don’t need planning permission.

Permitted development garden rooms need to comply with the following key criteria:

Be within the curtilage of a dwelling house. Meaning it needs to be a freehold domestic property incorporating a house and a garden.
The total area of buildings around the original house must not exceed 50% of the original garden area. This means your outbuilding can be really big, 50% of your garden, which included the front garden (the entire plot, not including the original house). Note however that non original buildings or extensions count as buildings and are deducted from the 50%.
However the garden building must not come forward of a wall forming the front elevation. The front elevation will normally be in the wall facing the main highway but not always.
Permitted development garden rooms can only have one storey.
The height cannot exceed 4 m in the case of a duel pitched roof. 3m for other roof forms. However if the building is within 2m of any boundary then the maximum height is 2.5m.
In all cases the eaves must not exceed 2.5m
The height is measured from the highest point of the existing ground level immediately adjacent to the new building. So if you are on a slope dramatic things can happen. There is also no restriction to dig down lower than the ground to increase the internal height as long as the exterior levels comply with the above.
House in conservation areas, world heritage sites or designated areas of outstanding natural beauty AONB, then some additional restrictions will apply. Generally theses properties still benefit from some permitted development options for garden buildings. Listed buildings do not have any permitted development rights.
Some properties have had permitted development removed under article 4 of the Town and Country Planning (General Permitted Development) Order 1995. Typically this applies to new build properties but may apply to buildings or areas of historic or architectural interest.
The use of the building must be “incidental to the enjoyment of the dwellinghouse” include the keeping of poultry, bees, pet animals, birds or other livestock for the domestic needs or personal enjoyment of the occupants of the house. It is not clear from this if you can sleep in it but the guidance does define that a purpose incidental to a house “would not, cover normal residential uses, such as separate self-contained accommodation or the use of an outbuilding for primary living accommodation such as a bedroom, bathroom, or kitchen.” A gym would be acceptable and it might be considered reasonable to have a shower and a WC. If your garden room is for working, it might be reasonable to have a kitchenette to make a cup of tea. If in doubt ask your local authority planning department.

More detailed government guidance on permitted development is available here.

Building Regulations

Some small buildings are exempt from building control. This can be beneficial in the case of a garden room where certain conditions make it very difficult or expensive to comply with full building regulations. One such item is foundations and especially if there is a proximity to trees. Building control generally like strip foundations, formed by digging a trench and filling it with concrete. The depth of that foundation would normally need to go below any tree roots in the ground. If the trees are close by that might be 2 or 3 m deep.

A couple of options could be screw piling or a raft foundation. Building control bodies don’t generally like these options but they can work really well for small buildings such as garden rooms.

Another advantage of exemption from building control is thermal performance. Current building regulations require very high thermal performance which might not be appropriate for a garden room. You may not be heating your garden room all the time or through the coldest days as you would with your home so may choose to reduce the insulation a little bit to reflect how you intend to use it.

For you garden room to be exempt from building regulations it must comply with the following:

A detached single storey building, having a floor area which does not exceed 30m2, which contains no sleeping accommodation and is a building:
(a) no point of which is less than one metre from the boundary of its curtilage; or
(b) which is constructed substantially of non-combustible material.
A detached building, having a floor area which does not exceed 15m2, which contains no sleeping accommodation.

The ‘constructed substantially of non-combustible material’ is an important point. This means not timber if you are within 1m of any boundary. In small gardens this can be a problem because you might loose a lot of potential space, it is also visually much more comfortable in some instances that the building is embedded into the surroundings.

Size and placement is the other consideration: 30 sqm if made from non combustable material and within 1 m of the boundary, 15 sqm in other cases.

If you do intend to use the building for sleeping in (even occasional) then technically you do need to comply with building regulations. You may also need full planning permission as stated above.

Materials

Timber buildings are generally cheaper than masonry, so consider the implications of the building regs exemption carefully.

Timber buildings are also better suited to hand driven screw piles and can be raised out of the ground, avoiding the use of concrete altogether. This is cost effective but does come with some compromises: Remember the height of the building under permitted development is limited (see above) so raising the floor may make the interior space too low.

Concrete foundations can be dug down and the floor can be flush between inside and outside. this maximises head height internally and grounds the building better visually. Hand driven screw piles can be used with concrete slab foundations, but you may need a few more so it does usually cost a bit more than for timber structures.

Summary

There are many factors to consider when thinking about and procuring your garden room. While it is possible to buy a prefabricated garden room for a few thousand pounds it may not give you want you want. Remember also that the use of your building may evolve over time and perhaps a more robust structure will be more accommodating to future uses.

Think about the size you want and where it will sit in your garden. Does it need to be less than 1m to the boundary? Do you want it ti look and feel solid or lightweight and temporary. Will it add value?

Please feel free to comment below if you have any questions or get in touch through our contact page