How (& how not) to build a base for your garden building

It is essential that your building is positioned on a flat, solid and stable base to take the substantial weight and to ensure that doors and windows hang correctly and open smoothly. The base should always be the same size as the footprint of the building and constructed of either concrete, paving slabs or a Garden Affairs Deck base. Making a base larger than your building is not recommended, even if you need a garden office with storage, this increases the risk of water ingress, so be sure that you know the exact size you need and if you are unsure then just ask!

Decide on a location

Always remember to make provision for roof overhangs all around your building when constructing your base near trees, walls, other buildings or boundaries. We recommend you leave half a metre all around your building for roof overhangs and maintenance including applying preservative treatment & paint finishes.

Bear in mind that if you site a small garden building directly under a deciduous tree the foliage will potentially drop directly onto the roof, which could block gutter outlets, causing rotting foliage to build up so it will be important to regularly clean off the roof and gutters if fitted.

If you plan to use your building (like a 5m x 3m garden room) all year round as an office or studio/workshop it's a good idea to consider positioning your building so that it receives plenty of natural light and consider placing windows where you have the best views of your garden. Also, give thought to positioning your building close to mains electricity and a water source if required.  

Trees too close to base of building
This customer has situated their base far too close to a tree and is practically overlapping their neighbour's boundary. A tree surgeon had to be employed to remove branches.

Guidelines for bases

We recommend the following three types of bases for our buildings: concrete, patio slabs or decking. The bases below are our recommended construction guidelines for buildings of up to 30m square. The information is provided for guidance purposes only.

Concrete Slab Foundation

 Mark the area and excavate to approx 150-200mm depth.

 Infill 75 -150mm with 40mm scalping (MOT 1) and compact using a vibrator plate, roller or tamper. Go over the whole area three times to ensure it is compacted.

 Build your shuttering so that the top of the concrete base will be approximately 3 – 5 cm above ground level. The inside dimensions of the shuttering should measure the exact dimensions of the building. Ensure the shuttering is level by placing a datum stake in the centre of the base to the required finished level and then taking all your spirit level measurements from that central point. You can check the base is square by measuring the diagonals and ensuring they are of equal length.

 A DPC (damp proof course) should be laid before pouring the concrete. This is to prevent moisture and dampness from rising up through the base.

Pour your concrete and tamp down to give a level even surface - for buildings over 16 sq m we recommend fibre or steel mesh reinforcement.

You will require between 75mm – 150mm depending on the size of the building.

 A gravel border of approximately 300 - 500mm will help prevent rain from splashing back onto the walls and so prolongs the life of the building.

If you are having electrics/water you should consider laying the mains supply now.

Example of a quality concrete base

Quality Concrete Base

And now for...totally Unsuitable Bases

There are no prizes for guessing what is wrong with these bases below. But just in case you don't spot the problems, we will guide you through!

Base too small

This base for a corner building has not been properly measured and is too small.

Base too small
Above this base is also too small because not enough room has been allowed for the protruding interlocking corners of the customer's log cabin. The building could not be built until the restrictive posts were removed.

Base not flat
This base had a lump of concrete poured on top of it which made it completely uneven, (probably due to a sloppy builder who couldn't be bothered to dispose of the excess concrete).  Note: The floor joists of a building need to sit directly on top of the concrete and haven’t a hope of being flat on a base like this.  Always ensure you use a spirit level to check that your base is completely flat and level.

NB: If a base is found to be more than 20mm out of level we may be unable to install your building. Should the base be out of level to such a degree that our installation team deem it is unfit for the supplied building, we will be unable to complete the installation on that day and a charge will be made.

Totally unsuitable base
We are not sure where to start with this one! Suffice to say it is totally unsuitable and consists mostly of bare earth with a few concrete slabs around the sides it will not hold the weight of a substantial timber building.

Uneven patio slab base

Above, here is an example of an existing patio base that the customer planned to use for their log cabin. Most patios are designed with a fall for water runoff (just like this one) and are therefore NOT suitable for a building base. As you can see the customer did not check that the base was level.

Read below how to make a patio base to sit a building on.

View our base work services

Base work

How to construct a Paving or Patio Base for your building

Mark the area and ground excavation to approx 150 mm depth.

Infill 75 -150mm with 40mm scalping (MOT 1) and compact using a vibrator plate, roller or tamper. Go over the whole area three times to ensure it is compacted.

Lay the paving stones on a mixture of semi-dry grit sand & cement 4:1 ratio to approximately 50mm deep and level as you go. (The mix should just have enough moisture in it to stick together when squeezed – this will form a very strong layer)

You need to ensure your paving is even and level and for under the building, it is best to use flat rather than riven (textured) slabs.

If an electric supply is required the main supply should be laid at this time.

Garden Affairs Deck Bases

If you would prefer not to build in concrete or patio slabs, an eco-friendly solution is our deck base system. What's more, if your ground is very uneven or sloping or you need to be careful with regards to tree roots or trees under preservation orders then a raised deck base is the ideal solution. Find out more about our deck bases and pricing.

Our deck bases are constructed of C16 pressure-treated timber and are supported on completely rot-proof recycled plastic posts or screw pile foundations, or a combination of the two.

Deck base for garden building

 Below is a Garden Affairs deck base raised from the ground to make the most of the beautiful surrounding views.

 Deck base
Raised deck base


We are often asked if a plastic grid base system is suitable for our buildings. The short answer is no! These types of plastic bases infilled with pea shingles probably work fine for a temporary children's play hut, but they are not designed to take the weight of a substantial building and will likely sink, taking your building with it! We will not guarantee any building built on this type of base.

So, now you have all the facts about how to build a tip-top base for your new garden building that will last the test of time!

Talk to an expert

Speak to one of our friendly advisors about a base solution for your new garden building.

Contact us