A lovely garden room in Chester with large paved area to front and side

Garden Room and Garden Office Foundations

Garden Room and Garden Office foundations are possibly the most important consideration when looking to install a garden room that will last a lifetime.

So, if you’re thinking of investing a fairly substantial sum in a new garden room or garden office, you will no doubts be interested to know the pros and cons of various types of garden room foundations. In this article, we try to answer the most important questions around garden room foundations, so read on to learn more..

Garden Room Foundations - What should you consider?

If you’ve done some research already, you’re probably aware that there are various types of foundations that may or may not be suitable for your new outdoor garden room. There are of course, claims and counter claims as to the most suitable foundation types, and it can be a nightmare for someone who is not familiar with building principles to understand what should and should not be done.

Indeed, some garden room suppliers will go to great lengths to persuade you to engage your own contractor to carry out this important work, as foundations are the bit most likely to cause long term problems if they aren’t done correctly, and they would rather that liability rests elsewhere.

So beware, if you agree to sub-contract your garden room foundations to another builder and things go wrong, your garden room supplier will use every excuse to walk away and their warranties will most likely be unenforceable..

Our  Garden Rooms and Offices include the full installation of foundations and base 

Let’s move on now and consider what the most important considerations are when assessing the most suitable type of garden room foundations for your project:


If you were installing a lightweight shed or summerhouse, you may be able to use slabs bedded on sand and dabs of concrete. However, a well built garden room is a different prospect altogether. Even a moderately sized 4m x 3m building from Rubicon will weigh around 4 tons, and if the foundations are incorrectly built, your building will slowly sink into the ground. The total weight of the garden room is therefore a very important consideration.

Type of Ground

Is the ground heavy clay? Sandy? Full of stone? Water-logged? Different types of ground are an important consideration in identifying the type of foundation most suited to your garden room installation. For example, heavy clay soil can slump and heave as it goes through the seasonal drying and wetting process. Waterlogged ground may need deeper foundations. Therefore, your foundations need to be designed to cope with these, and other variations.


People don’t always realise that different types trees can seriously undermine the foundations of a garden room. Some, such as leylandii are deep rooted and very thirsty, so they severely dry out the ground nearby. Others such as Oak trees are relatively shallow rooted but extend laterally to a great distance. There may also be Tree Preservation Orders (TPOs) on certain types of tree and so we need to ensure that the design of your foundations does not cause the tree to die off


If you’ve already got flat, level ground to mount your garden room foundations on then that’s a bonus, but most of the time the ground is either uneven or can slope quite severely one way or another. The type and design of the garden room foundations will need to take account of this. Excavating large amounts of soil is both an expensive and a very messy business involving mini-diggers ripping up your garden, so we prefer to use foundation design methods which avoid this problem wherever practical.

A picture of a garden room garden office foundation system

What types of Garden Room Foundations are there?

So having considered the most important questions and issues faced when building a garden room or any similar outbuilding, let’s now look at the types of garden room foundations and their pros and cons:

Plastic Grid Systems

Plastic Grid Systems are amongst the cheapest options available and some even claim to be able to support a substantial weight. However, they are only as good as the substrate they are mounted on, so chances are that, if the ground moves, so will they. However, they may be reliable enough for lightweight shed structures.

Concrete Paving Slabs

Again, unless supported by a good substrate of at least 6” of well compacted hardcore and cement bedding Concrete Paving Slabs are unlikely to remain level. They are also prone to subsiding, particularly from the impact of nearby tree roots, and the subsequent removal of moisture from the ground. They are therefore an unreliable option for any heavy building and will also require levelled ground.

Concrete Pads

If built properly, Concrete Pads can be a good solution. Solid blocks of concrete usually 600 x 600 x 600 deep can enable a solid base. However there’s a lot of concrete used, and digging them is a very labour intensive process.

Concrete Slabs

The main issue with a Concrete Slab is trees and ground stability. Too many large slabs are put down with thin layers of substrate and concrete. If using this method, a minimum hardcore of 6” MOT and 6” fibre reinforced and/or rebar strengthened concrete should be laid.

If trees are nearby or the ground stability cannot be guaranteed, we would recommend a Raft foundation is built using a pre-formed mesh framework. However, even the best laid slabs can subside and crack (as many old garages do), and the cost and excavation needed can be messy and prohibitive.

Old Garage Slabs

Very often, an Old Garage Slab will be suitable for a garden room foundation as they are designed to take considerable weight. The main things to look out for when assessing their suitability are:

1) Are there any major cracks in the slab (Minor ones are usually ok)?
2) Are there any signs that the slab has sunk out of level (even if it hasn’t cracked)?
3) Are there any signs of major cracks in the walls?
4) Are the garage doors jammed and difficult to open?

Remember also that, if the subframe of the garden room or office is made from timber (even if it’s treated), it should NOT be simply placed on the slab and built on. The subframe should be raised off the slab an inch or so to ensure that the timbers are not constantly sitting on wet ground, as this will eventually rot the subframe timbers, even if they have been treated.

Ground Screws

Ground Screws are a good solution if installed correctly. They will take the weight and spread the load. The problem we’ve found is that the installation companies offering them are only subcontractors, and so any companies using their services are reliant on them sticking around to honour any guarantees given. We’ve always been very nervous of this and so have resisted the temptation to use them as we would be left with any residual liabilities if the worst came to the worst.

Pile Foundations

Pile  Foundations are our preferred method with proven reliability in excess of the 10yrs we’ve been using them. These are similar to Ground Screws, but use a concrete and steel mix to form a concrete mini-pile around 600-700mm into the ground. As an example, a 4m x 3m building has around 20 of them positioned at strategic points to take the weight of the building and the floor. Our system is similar to the pile system that supports the old buildings of Venice - the load is spread across multiple points. It’s just like lying on a bed of nails where the nails will not penetrate the skin because the weight is evenly spread.

The advantages of this system are:

  • the load is spread across multiple points
  • it’s suitable for different types of ground conditions
  • it’s adaptable to sloping and undulating ground
  • it’s not affected by tree roots
  • it doesn’t damage tree roots
  • it takes substantial weight such that a car can be parked on the finished base
  • there are no large machines needed for excavation
  • it’s cost effective and minimises the use of concrete
  • it’s an in-house system fully designed and installed by us, so we are not reliant on the long-term guarantees of external subcontractors
A garden room in Chester, Cheshire with a hot tub and sliding aluminium doors

Conclusions - so what’s the best foundation for a garden room?

As can be seen, there are lots of potential foundation systems available, but not all are suitable for a garden building weighing many tons such as those supplied by Rubicon. When building a house, the two most important elements for longevity are the foundations and the roof.

It’s the same with a garden building - if it’s built on solid foundations it will last a lifetime. In the worst case scenario if something does go wrong with any building, everything above ground is easily repairable, but if your foundations fail…

At the end of the day the choice of garden room supplier is yours, and each will offer, and probably recommend different solutions to the problem. Hopefully, this article will help you make an informed choice and ensure that your garden room foundations are built to last, whoever you choose.

A final few words of advice - beware any firm who tells you it’s cheaper or easier to get your own foundations installed by a local builder. All they are doing is passing on the liability, for the most likely point of failure, to you. 

Find out more about garden rooms & offices with Rubicon Garden Rooms

For more information about our award-winning garden offices, contact us and we’ll be more than happy to help.

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