ᐅ Existing basement, a new full basement, or no basement at all?

Hello everyone,

My wife and I are increasingly planning to build a new house on my grandmother’s property.

The plot is about 770m² (8300 sq ft) and currently has a house from 1956 with a 9x9 meter (30x30 ft) footprint plus an extension and a garage.
My grandfather built the house back then. The basement is quite low and not very deep below ground level (approximately 80cm (31.5 inches)).

We have a development plan from the 1960s in Sütterlin, which allows for 1 full story with a 50cm (20 inch) knee wall.
However, the building authority has indicated that neighboring city villas with 2 full stories and a shallow hipped roof represent the upper limit of what is possible.

What concerns me somewhat is the question of the basement.
I have read the so-called 11% basement rule, but I don’t think I fully understand it.
I can follow it in the case of a hillside house.
But does it mean that if you have a (basically) level plot, you would never build a basement?
Instead, you would rather increase the footprint of the house and place the typical utility room on the ground floor?

How would it be if there is already a hole from the demolished existing house?
I actually see the old basement as unusable— it’s not deep enough and unfortunately not in great condition anymore.
The area, though, would roughly correspond to what we are envisioning.

In our case, the current plan is that the basement would have, besides the technical/ laundry room, a small storage/ pantry area.
Also important is a large workshop (20m² (215 sq ft)) and a living room (20m² (215 sq ft)) for office or guest use.

How can I calculate whether it would be more economical to level the existing basement and increase the house footprint instead?

I always thought that it gets more cost-effective the closer you get to building a cube — minimizing exposed exterior wall and roof area.
The number of floors is limited, after all.

Best regards

FitoCari schrieb:

But does this mean that with a (pot) flat plot, you would never build a basement?

Not exactly. Nowadays, a regular homeowner would almost never do that because it’s simply too expensive. Either a utility basement, which you usually don’t need as large as the house’s footprint (since it’s cold and uninsulated/unheated), or a living basement – but who wants to “live” in a basement that’s often more expensive than above-ground living space.
So it’s only done if money is no big concern.

FitoCari schrieb:

Rather, you increase the house’s footprint and place the technical room on the ground floor?

Yep, because that only costs a few thousand euros.

FitoCari schrieb:

How can I calculate whether it’s better to level the existing basement and instead increase the house’s footprint?

You could simply ask pragmatically what demolition and filling would cost. A construction or demolition company should be able to tell you. The new basement construction costs after demolition can also be reasonably estimated.

FitoCari schrieb:

In our case, the current plan is that the basement would house the technical/laundry room plus a small storage/pantry area. Also important are a large workshop (20m² (215 sq ft)) and a living space (20m² (215 sq ft)) for an office/guest room.

A workshop could be set up in a garage with a separate workshop area (why 20m² (215 sq ft)?). Technical room and storage (what exactly are you storing that needs a large room?) could go on the ground floor – which would make the upper floor somewhat larger and suitable for the office (do you really need 20m² (215 sq ft) for that?).
A basement with these requirements, especially including living space, has to be within the thermal envelope. For an 80m² (860 sq ft) basement, you’re easily looking at the first €200,000 and more...

Hello Nordanney,
thank you for your quick response.
I understand the two extreme cases.
If you want a 100% residential basement (comparable to a granny flat), it’s uncomfortable in the basement and more expensive than above ground.
If you plan a 100% utility basement (e.g., storage, technical room, washing machine), it’s uneconomical because the area is far too large.

That leaves the partial basement. Only the few square meters for utility rooms and the space intended as living areas are moved upstairs.
I am somewhat skeptical of a partial basement structurally. Perhaps this is because in my last rented apartment, problems occurred exactly at such a transition between the underground garage outside the building footprint.

So, a utility room (technical room) could be moved upstairs and integrated there (only slightly) more expensively.

I don’t want to discuss the sense of a 20m² (215 sq ft) workshop. It’s a hobby workshop. And hobbies don’t have to be rational.

The 20m² (215 sq ft) office/guest room came about from the previous plan. However, the basement question doesn’t depend on whether it’s 16 or 12m² (172, 129 sq ft) or 20m² (215 sq ft).
A 10m² (108 sq ft) storage room quickly fills up with skis, suitcases, hiking boots, jars of jam, and currently raclette equipment and Christmas decorations.

FitoCari schrieb:

Then there’s the partial basement. Just bring up the few square meters for a utility cellar and the space intended as living area.
Structurally, I’m rather skeptical about a partial basement.

Structural concerns are not an issue. However, a partial basement (used as a utility cellar) is noticeably more expensive per square meter than a full basement.

FitoCari schrieb:

So you would move a utility room (technical room) upstairs and integrate it there (only slightly) more expensively.

Exactly.

FitoCari schrieb:

I don’t want to discuss the purpose of a 20m² (215 sq ft) workshop. It’s a hobby workshop. And hobbies don’t have to be rational.

No question about the purpose of the workshop, just the size. As I said, you could also build a standard double garage and attach your small room to it. A 6 x 9 m (20 x 30 ft) space already fits a lot (including workshop-related items) and could for example be extended by another 3 m (10 ft). Or just a 6 x 6 m (20 x 20 ft) garage with a 3 x 6 m (10 x 20 ft) workshop “extension” attached.

FitoCari schrieb:

A 10m² (108 sq ft) storage room quickly fills up with skis, suitcases, hiking boots, jam jars, and (currently) raclette equipment and Christmas decorations.

Or those things end up in an attic with possibly sufficient headroom, which is enough for items that are only occasionally used.

All food for thought. If the budget allows, a basement is always great. I’m personally happy to have plenty of space in my basement—it’s an older building where the basement was already established (also for the oil tank, heating room, laundry).

Thank you for the suggestions.
It can’t be inferred from my initial post that I have already considered these points. Please excuse that.
For example, moving the workshop into the garage... I have already thought about this and did not raise it as a question here.

What interests me more is the mixed cost calculation if an existing basement is available.
No Basement:
- Existing basement must be demolished and filled in.
- A slab on grade is required anyway, whether at 1m (3.3 feet) depth or at ground level.
- However, the slab needs to be about 20m² (215 sq ft) larger to gain more space on the ground floor and first floor. (just a hypothetical area for now)
- The roof area must also be increased accordingly by 20m² (215 sq ft).
- Are masonry walls cheaper than basement walls? Plaster finish versus waterproof concrete ("white tank") method.
- Basement stairs would no longer be needed.
- Interior fittings for office/guest/workshop space remain the same, regardless of whether on the basement, ground floor, or first floor.
- Escape routes from the basement are no longer necessary.
- A sewage lifting station would not be required. (Although I believe I wouldn’t need one even with a basement. I am aware that this assumption might not be very reliable.)

How can the calculation be derived from this to determine whether a full basement “pays off” compared to relocating the space to the ground and upper floors?
I still probably have many assumptions as a layperson and no real sense of how to balance the costs.

Maybe some additional information about the plot:
The soil is apparently very clayey. This means that everywhere construction will take place, the soil must be replaced.
This was the case with the newly built town villas nearby.
Therefore, I see an advantage in the existing house. If we keep the same or a similar footprint, we would save this effort. The excavation for the full basement is already half done.
The plot is quite level. Not completely. But I would categorize it more as level rather than a sloped lot, where the basement would open halfway onto a terrace.

FitoCari schrieb:

What I’m more interested in is the mixed cost calculation, in case an existing cellar is available.

What does a mixed cost calculation mean to you? If you build the cellar new according to your wishes and need to demolish the current cellar, it will be about 2-3 times more expensive than building without a cellar.