ᐅ Building a Living or Hobby Basement Underground – What to Consider?

Hello everyone,

We are currently planning our outdoor area and want to build a hobby room underground.
On top of it, there will be a layer of soil so grass and flowers can grow, as well as a garden shed that will be used as a sauna.
The room should be completely embedded in the ground and covered with soil.

The dimensions are planned to be 4x5 meters or 4x6 meters. Heating and electricity will be supplied from the main house, which is not a problem.

My question is what should be considered here. I want to build it myself and I am quite handy.

So I have some basic questions like:

Thickness of the base slab?
30 cm (12 inches) masonry with hollow concrete blocks, or other materials/thickness?
Waterproofing on all sides against cold and moisture?
How to build the ceiling?
Entrance maybe next to the sauna?

Light should come into the room through a wide window with a light well, or on the opposite side, underwater in the adjacent pond through a glass panel.

I hope you can provide information, help, and tips to make this project possible.

I have already looked online without success for similar projects but found nothing. Unfortunately, I only find information about underground cellars for storing fruits and vegetables.

Attached is an amateur sketch to illustrate the whole idea.

Best regards

Edit: Unfortunately, the photo of the sketch is too large, and I can’t reduce its size using my phone. Let’s try without it for now, and otherwise, I will see if I can resize it on the PC and make it available.

You want to build a single basement room that a) can be heated and b) can withstand the pressure of a pond?

I believe you don’t fully understand what you are getting into: floor sealing, structural engineering, drainage, use as a habitable room without a building permit/planning permission… basically everything has to be done from scratch again, just for a benefit that only covers a few square meters.
I don’t think this is allowed to be built freely like that.

Webmaster-uk schrieb:
My question now is what needs to be considered here.

- Structural engineering to ensure the roof doesn’t collapse. A structural engineer can help with this.
- Moisture protection, which depends on the soil conditions and what is necessary.

Then various details will certainly need to be planned:
- Installation of utilities, again with moisture sealing,
- Drainage (stairs, doors, light wells),
- Ventilation, heating.
This should not differ from a typical above-ground living space, just more complex.

A building permit / planning permission is probably required.

Between the roof and the ground, a root barrier layer should be installed (similar to a green roof, so nothing complicated).

After all the planning, there will also be important considerations during construction, especially the sloping of the excavation pit (depending on the soil conditions).

By the way: What is this for?

What is the budget for the project?

Thank you very much for your previous answers.
The plan is to build a fitness room.

I believe installing the heating pipes will be the least complicated part. The pipes will come from the main house, and the necessary preparations have already been made.

In principle, it should be possible to build it just like a regular basement, except perhaps with a reinforced ceiling due to the constant load from above, right?

I haven’t thought about the budget at all yet, since I don’t know how complicated or time-consuming it will be. Otherwise, you can expect typical cost per square meter for a basement.

A friend recently built and got the basement done for 20,000 euros for a floor area of 80 sqm (860 sq ft). That would be 250 euros per sqm (about $250 per 11 sq ft). And that was all done by a company. I would do it myself. So for a size of 4x5 meters (13x16 feet), that would be around 5,000 euros (about $5,000) if converted.

The question is basically about how and what needs to be considered.
So, which ceiling for the slab, insulation, what kind of blocks, ceiling with beams and pouring or other options?

I’m not looking for structural calculations or anything like that here, but tips, help, and support for planning the individual steps and considerations.

Regarding a possible building permit (which I don’t really expect will be necessary), I can just inquire at the local authority once the project is finalized and planned. So the permit shouldn’t be an obstacle.