How to build a concrete base for a Garden Room – a practical guide
- 1. Why choose a concrete foundation for a garden room?
- 1.1. Maximum durability and load-bearing capacity
- 1.2. Protection against moisture and rodents
- 1.3. Stable ground without cracks or subsidence
- 1.4. Compliance with regulations and formalities – also in Germany and the United Kingdom
- 1.5. Ready-to-use floor
- 2. When is it worth using a concrete slab?
- 3. How to make a concrete foundation for a garden room? Step-by-step instructions
- 4. Summary
- 5. Frequently asked questions (FAQ)
Are you planning to build a garden room and wondering what kind of foundation would be best? Whether it’s a small tool shed, a gazebo with a terrace or a solid year-round house, a properly prepared concrete foundation guarantees durability, safety and comfort of use. In this guide, we explain when it is worth choosing a concrete foundation slab, what its advantages are and how to build a concrete foundation for a garden shed step by step – even on your own. If you are looking for practical tips and want to avoid mistakes, this article is for you.
Why choose a concrete foundation for a garden room?
A concrete foundation slab is the most durable and versatile solution for garden houses – regardless of whether you are building a small tool shed, garden room, a summer house or a year-round place to work and relax. Below are some specific reasons why it is worth investing in one.
Maximum durability and load-bearing capacity
A concrete foundation is an investment for decades – a well-made slab can last over 50 years without the need for renovation. It will easily support:
- a log cabin,
- a metal workshop,
- heavy point loads (e.g. tool racks, a garden stove, water barrels).
For comparison: a point foundation or a foundation made of paving slabs may, over time, sink or warp under heavy loads, leading to problems with the doors and walls of the house.
Protection against moisture and rodents
A concrete foundation slab also acts as an effective insulating barrier – it protects against:
- moisture rising from the ground,
- mould and rot on the floor,
- mice, moles or insects getting in from underneath the structure.
In combination with insulation foil and adequate ventilation, this is the way to a dry and healthy garden shed even in more difficult conditions.
Stable ground without cracks or subsidence
Unlike sand or loosely laid gravel, concrete provides a uniform, compact and perfectly level surface that:
- does not shift over time or with frost,
- does not collapse under the weight of the house,
- eliminates the problem of warped walls or crooked doors.
This is particularly important in houses with precisely fitted elements, such as the block-built houses in our range.
Compliance with regulations and formalities – also in Germany and the United Kingdom
In many European countries, including Germany and England, the construction of a garden house – especially one with a larger area – is subject to formal requirements. For houses with an area of more than 30–35 m², the following is very often required:
- building permit (German: Baugenehmigung, English: Planning Permission),
- and a foundation permanently connected to the ground, ensuring adequate load-bearing capacity and protection against moisture.
Germany:
- In most federal states, garden sheds larger than 30–40 m² or with a functional purpose (e.g. workshop, sauna) must be built on a permanent foundation – a concrete slab is often recommended.
- For projects with connections (e.g. electricity, water), local authorities (Bauamt) require a permanent foundation that allows the installations to be carried out in accordance with DIN 1055 and the regulations on ground moisture.
United Kingdom:
- In England, if a garden room is more than 2.5 m high and is located less than 2 metres from the property boundary, it requires a permit and a suitable foundation (concrete base is recommended).
- For larger structures (over 15 m²), especially if you plan to connect to the mains (electricity, water), local councils require a permanent and stable foundation that does not settle and meets moisture requirements (moisture-proof layer recommended).
Does a concrete slab meet these conditions?
In both Germany and the United Kingdom, a concrete foundation slab is the most commonly accepted form of foundation for structures:
- it is resistant to temperature changes, moisture and frost,
- it allows for easy installation of utilities in accordance with technical regulations,
- it ensures stability even on difficult soils (e.g. clayey or waterlogged).
Ready-to-use floor
A concrete slab can successfully serve as a finished floor – all you need to do is smooth it properly and, for aesthetic reasons and easier cleaning, you can paint it with concrete paint. This gives you the following benefits:
- a durable, dirt-resistant surface,
- lower costs, as you do not need to lay additional flooring,
- greater convenience, e.g. for DIY, assembling shelves or storing equipment.
This solution is ideal for garden rooms, tool sheds, workshops or garden saunas, where functionality and resistance to moisture and intensive use are important.
When is it worth using a concrete slab?
Not every garden structure requires a solid concrete slab right away, but in many cases it will be the best and sometimes the only sensible solution. Here are some situations where a concrete foundation is essential for durability and safety:
1. For larger structures (over 10–15 m²)
If you plan to build:
- a garden house with windows and a terrace,
- a tool shed, a bicycle shed,
- a workshop or hobby area (e.g. with a vice, table drill, workbench),
…a concrete slab will ensure stability, even support and full load-bearing capacity, regardless of the weight of the structure and the intensity of use.
2. On poor ground
If the plot has:
- sandy, loose soil,
- damp turf,
- uneven ground or root remnants,
…traditional point foundations may sink or shift. A concrete slab acts as a binder that levels out uneven ground and distributes the load evenly over the entire surface.
3. In areas exposed to moisture and precipitation
If the plot is located:
- in a valley or near water reservoirs,
- on clayey, impermeable soil,
- in a region with high precipitation (e.g. southern Germany, Wales),
…a concrete slab acts as a waterproof barrier – it protects the structure from moisture rising from the ground and reduces the growth of mould. In combination with a layer of insulation foil (DPM), it prevents costly repairs and damage to the wood.
4. When you plan to store heavy items
A shed is not always just for storing rakes and watering cans. If you plan to store:
- shelves with tools and gardening equipment,
- a concrete barbecue, stove or water barrels,
- a wood cutting machine, press or compressor,
…a concrete slab will support heavy loads without sinking, cracking or deforming.
5. If you plan to install utilities (water, electricity, drainage)
Many garden shed owners eventually decide to do more than just store equipment – they install lighting, sockets, water supply and even heating. In such cases, a concrete foundation slab offers a significant technical advantage already at the preparation stage.
Thanks to concrete:
- you can lay and hide cables and pipes in the ground before the concrete is poured,
- you can easily plan passages under the walls or in the central part of the garden room,
- you avoid running cables along the walls or outside, which looks unsightly and exposes the installations to damage and moisture.
In addition, this type of foundation facilitates technical approval in countries such as Germany and England, where a design without a foundation integrated into the ground may be rejected at the application stage.
Tip from a BHB expert:
If you are planning a cottage with LED lighting, a jacuzzi pump, a garden sink or even an electric bike charging station, a concrete foundation will allow you to install everything aesthetically and safely.
Below you will find a brief comparison of the most common applications for concrete foundations. This table will help you assess whether such an investment is really worthwhile and technically justified.
| Situation | Is it worth using concrete? | Why? |
| Cottage <6 m² for tools only | Not likely | Paving slabs or point foundations are sufficient |
| 6-10 m² bungalow on light ground | Yes, it is worth it | Concrete will provide stability, especially in uneven or waterlogged terrain |
| Cottage >10 m² (with windows, terrace) | Definitely yes | Durability, compliance and load-bearing capacity of the structure |
| Workshop with heavy tools (drills, shelving) | Definitely yes | Only concrete can cope with high point loads |
| Cottage with electricity or water connection | Yes | Concrete enables installation to be carried out in accordance with standards |
| Site with high humidity/intensive rainfall | Yes | Concrete acts as a barrier against seepage and dampness |
| Willing to make a finished, durable floor (without boards) | Yes | Concrete can be smoothed, painted and used without additional finishing |
| Temporary construction for 1-2 seasons | Not | Too expensive – better bet on cheaper and mobile foundations |
How to make a concrete foundation for a garden room? Step-by-step instructions
Are you planning to build a concrete foundation for a garden room, tool shed, garden sauna or mini workshop yourself? That’s a great decision – a concrete slab is a durable, safe and versatile solution. Here is a proven step-by-step guide that will take you through the entire process.
1. Marking out the outline
First, mark the exact dimensions of the planned foundation – preferably with a mason’s line and pegs.
Tip: the foundation should have exactly the same outline as the shed (without ‘edges’) to prevent rain from bouncing off the slab onto the walls.
For 3×3 m or 4×4 m models, which are among the most popular models in our offer, it is worth adding a few centimetres of assembly tolerance, e.g. 305×305 cm.
2. Preparing the ground and foundation
Remove the top layer of soil to a depth of about 20–30 cm (and even deeper in the case of peaty or clayey soil).
On the prepared ground, lay a 10–15 cm layer of crushed stone or gravel (e.g. type 0–31.5) and compact it thoroughly using a mechanical compactor.
BHB expert opinion:
The foundation is often an underestimated stage, but it is essentially what determines the durability of the entire slab. Poorly compacted soil = cracked concrete after winter.
3. Insulation foil
Spread insulation foil (min. thickness 0.2 mm), known as a damp-proof membrane (DPM), over the compacted gravel. This prevents moisture from rising from the ground and protects the structure of the house – this is particularly important for wooden houses.
Interesting fact: well-laid insulation foil extends the life of the foundation by up to 10-15 years and significantly reduces the risk of mould in the wooden structure.
4. Reinforcement
Lay out the reinforcement mesh (e.g. 6 mm rods, 15 × 15 cm mesh) a few centimetres above the foil (e.g. on plastic spacers or bricks).
This will ensure better load transfer and prevent the slab from cracking.
Reinforcement is particularly important if the house:
- has a large roof or canopy,
- contains partition walls or a mezzanine,
- is located in an area exposed to snow loads (e.g. mountains, northern Germany).
5. Concrete screed
Pour a mixture of C25/30 concrete (for a standard shed – approx. 10–15 cm thick, for larger ones – up to 20 cm).
Ready-mixed concrete works well, but you can also mix it yourself (1 part cement : 4 parts gravel : 2 parts sand + water).
If you do not know how much concrete you will need, ask our advisor.
6. Levelling and smoothing
After pouring, spread the concrete with a shovel and level it with a patch (e.g. a mason’s level).
For better adhesion and safety, you can brush the surface (on fresh concrete) to achieve a non-slip effect – ideal for tool sheds or wet areas near saunas.
7. Concrete curing
Concrete does not like to be rushed, therefore:
- cover the freshly poured slab with foil or non-woven fabric for at least 72 hours,
- sprinkle the surface with water for 3-5 days in hot weather.
The slab reaches its full strength after approx. 28 days – only then can the shed be erected.
Summary
A concrete foundation is more than just a base for a garden room – it is an investment in stability, comfort and peace of mind for many years to come. With a well-made slab, you can be sure that the structure will not settle, crack or require repairs after the first winter.
Whether you are planning a small tool shed, a functional workshop or a garden sauna with installations, a concrete base gives you full control over what you are building and how long it will work without problems.
If you have any doubts about how to match the foundation to the selected model of the garden room or the ground conditions, contact the BHB experts. We will advise you, calculate and suggest the best solution for your garden.
Frequently asked questions (FAQ)
1. Does a garden room need a concrete foundation?
Not always, but if the garden room weighs more than 300-400 kg or is to be used all year round, it is definitely worth placing it on a concrete slab.
2. What kind of foundation for a garden room on an allotment?
A point foundation is often sufficient, but if you want durability and moisture resistance, choose concrete. Remember to check the allotment regulations before laying the foundation.
3. How much does a concrete foundation for a garden room cost?
Prices depend on the thickness of the slab, the region and the materials. Estimated cost: from £100 to £200 per m² including labour. The cheapest option is to do it yourself.
4. Can I make a concrete foundation myself?
Yes, if you have the basic tools and time. The foundation for a small 3×3 m garden shed can be prepared in 2 days.
