ᐅ Outbuilding as a Basement Substitute: Are There Legal Issues?

I am currently planning a new build. For several reasons, I want to avoid a basement (groundwater, costs, avoiding stairs). Therefore, I obviously need compensatory floor area. I have been considering an extension for this purpose.

The extension would include: a laundry room, storage, a technical room if possible (is this allowed under building regulations?), and possibly a hobby room.

However, an extension built to the same quality as the main house is simply too expensive.

I have thought about alternatives: For example, I contacted a supplier of precast concrete garages. According to them, they also often sell outbuildings that are additionally insulated and can be used for this purpose. The "building" would be thermally separated from the main house (by a door) but connected to it. It would have 10 cm (4 inches) Styrofoam insulation. It features a flat roof, which should be greened and possibly partially used as a rooftop terrace. It should also include windows, electricity, and heating supplied from the main house.

When I asked a house-building company, they generally advised against it and stated: "It won’t be cheaper once you include the interior finishing."

I believe that since basements can be offered for well under €1000 per sqm and the quality requirements regarding thermal insulation, etc. are not as demanding, this should actually be an affordable alternative.

My questions are:

• Are there building regulation or technical issues with this?
• Is an outbuilding a privileged structure, and is it allowed to be built within the setback areas?
• Would other technical solutions be more cost-effective, such as precast concrete components (individual walls instead of a finished "box"), cast-in-place concrete, solid precast elements like expanded clay blocks, or conventional masonry?
• What are your experiences or opinions on this matter?

I have searched but have not yet found a thread that deals with this topic. I would have thought that this is a common issue when building without a basement.

Thanks in advance,
Andreas

Hello,

venraij schrieb:

The extension should include: laundry room, storage, if possible a utility room (is that legally allowed?), and maybe a hobby room ... Now, an extension built to the same quality as the main house is simply too expensive for me ... For example, I asked a supplier about precast concrete garages ...
When you ask a home builder, they usually advise against it immediately and say: "It won't be cheaper once you include the interior finishing."

That opinion is correct. You can only save money in this area if you build the extension walls yourself; your supplier would then need to pour the foundation slab and provide the structural calculations, and the rest is not rocket science ... as long as you forgo making it accessible, which — by the way — is not allowed within the required 3.00 m (10 feet) setback area anyway, and don’t place it right on the property line.

venraij schrieb:

I think you can get basements offered for well under 1000 €/sqm ...

Yes, I remember those ads ... they usually mean four unconnected walls (total 50 sqm or 538 sq ft) and nothing else ....

venraij schrieb:

Is an outbuilding a privileged structure, can it be built within the setback distances?

Since the extension must be insulated, it is not permitted to place it on the building boundary. In case of doubt, the relevant state building regulations provide guidance.

venraij schrieb:

I have searched but have not found a thread dealing with this topic.

By now, you should have learned quite a bit about setback requirements.

Best regards, Bauexperte

Hello,

The room is to be considered as living space according to building regulations, as you described, and therefore it is not privileged.

Consequently, the room itself must have a 3m (10 feet) distance from the property boundary or this must be secured by a building encumbrance on the neighboring property.

Exceptions confirm the rule; for more information, I need plans from the building project.

Best regards,
Dirk Grafe

Okay, thanks already for the information. Building the extension as an attached structure might not be that important or practical, since there are restrictions regarding windows and this would counteract the purpose of better lighting.

After reviewing the building regulations, I came to the conclusion that a building without any habitable rooms qualifies as a privileged building.

However, even if I forgo the attached structure, the issue remains with the very high insulation requirements for the extension. This does not necessarily make sense for purely utility space. Since financial constraints usually have to be considered in house construction, I still see potential savings here if certain compromises are acceptable. Especially since basements are generally not recommended in the planned development area due to high groundwater pressure.

The mentioned precast concrete supplier also said they frequently offer this solution. It makes sense from this perspective because I was offered finished basements with a footprint twice as large as that of the house, costing well less than half. This raises the questions:
a) Is such a solution permitted?
b) If yes, what limitations would apply— which rooms or facilities would not be allowed in such a building?
c) What other cost-effective alternatives exist?

I understand that if you want living space like in a residential building, you face the same costs, but does the same apply to less demanding utility spaces?

venraij schrieb:

After reviewing the building code, I have come to the conclusion that a building without any habitable rooms qualifies as a privileged building.

If you want to know the details, you need to check the state building code BB. In NRW, for example, non-privileged rooms also include technical rooms (heating) or unauthorized uses. That means if something is stored permanently in an attached garage that is not allowed to be stored there, the garage technically loses its privileged status due to the change or unauthorized use.

But even if I forgo the adjacent building, the issue of the high insulation requirements for the extension remains. That does not necessarily make much sense if it is purely storage space.

That probably depends on what you intend to store there.

Best regards,
Dirk Grafe