Garage on a slope / basement, L-shaped stones, erosion?

Hello and thank you very much for reading. I will try to describe our problem / challenge exactly, sorry if it ends up being 2-3 sentences longer and thank you for reading:

We found a super beautiful hillside plot for our house and are already in the middle of the planning phase with a civil engineer. The house is almost fully planned and only a few details are causing us difficulties – if only it weren’t for the garage/carport.

The plot slopes down about 2.5 meters from the street and then runs fairly straight. Because of this, we have to build with a basement (semi-basement). Seen from the street side, the ground floor is at ground level; from the garden side, it is on the first floor, and the basement is at ground level. So far, everything fits. However, we still need to get the garage or a carport planned. The unit is supposed to go between our house and the neighbor’s garage, who built directly on the boundary (with a building easement). Legally, everything is clarified; we are also allowed to build up to the boundary and the neighbor agrees to an extension of the easement, so that we can build slightly offset.

But now we somehow have to get our garage “up to the first floor” and the whole thing should not cost an additional €20,000 (basement). Since we have enough space in the house, we do not need any more rooms – not even for garden tools or similar. Therefore, we would like to simply “fill with soil” and place the carport on top.

Our builder initially had the idea to take a carport, simply attach it to the house with the roof (house statics allow this), and on the other side support it with point foundations/pillars down to the natural ground. In between, a “simple backfilling with soil and stabilization towards the garden and towards the neighbor via a retaining wall (e.g. gabions)” should be done.

We ourselves had the idea to simply use L-bricks to retain the soil towards the neighbor’s garage (it has a basement, so we have to retain properly so the garage is not damaged) and also towards the garden.

He (the builder) has now withdrawn his proposal (gabions and point foundation). His structural engineer thinks this is uneconomical because the soil would have to be compacted in layers if cars were to be parked on top of the carport. In the end, it would be just as expensive as a basement. The L-bricks are also rejected by the structural engineer because a very massive foundation would be needed for them.

We have already found the structural engineer to be very difficult many times and cannot really imagine that it should be so hard to realize. Maybe a professional or someone with similar problems reads this and has a tip?

If our earthworks company is going to remove the soil anyway (only 60-100cm with us) and then prepare a base on which our basement along with the house (about 100-120 tons on 80 sqm) can stand, then it actually should not be a problem for them to prepare another 36 sqm (6x6m) next to it, so that the L-bricks can be placed there and hold. We would use the ones with 300cm height and set them 6 meters south (behind them nothing because the garden starts there) and 6 meters east (towards the neighbor’s garage to retain). The bricks weigh about 2 tons/m. If we then lean a carport against the house and mount it directly on the L-bricks on the other side, that should actually be rock solid. In the garage there could be about 6 tons of cars at worst (two Q7s; although we currently drive an A-Class – about 1.5 tons).

Can such massive L-bricks and usual backfill with paving on top actually carry this weight without sinking inunstably if not compacted?

Alternatively, wouldn’t a slab on point foundations down to the natural ground be significantly cheaper than making a whole basement room (6x6m)? Especially since we would again need a paved floor for that.

Are we just thinking too weird or has the structural engineer been a bit too cautious here?

I am grateful for any little hint (also links), currently the garage topic is quite stressful and we cannot imagine that we are the first hillside property owners who want to build a garage without a basement.

Best regards and many thanks in advance from the little dragon

I'm not a structural engineer, but just at the thought of putting a garage on L-blocks, my toenails curl up.

I mean - even if you find a structural engineer who will sign off on it - just do it. And report back to us in no more than 3 years about the settlement cracks, because your L-blocks, not connected to each other, will twist however they please and your garage will follow suit. Luxury building damage - and then the engineer is supposed to be the fool in court.

So, enough complaining, in my humble non-expert opinion there are two options:

1. Basement. The argument "enough space" doesn't count because you can (re)plan that and the costs for the basement are obviously about the same as for a proper foundation. So why put in a lot of effort for a ground-level foundation when the foundation including the basement is just as expensive!?

2. Extend the ground floor slab of the main house/ground floor, i.e. attach the garage slab like a cantilevered balcony that can carry two Q7s plus the garage. That poses two problems: a) thermal bridge/cold rib, i.e., this probably means increased thermal insulation effort and b) you'll probably need a lot of steel for that. Both will increase the costs.

Long story short ... I would extend the floor compaction of the basement to the boundary and build a garden storage room with a door to the garden under the garage. Then you have ONE complete foundation under the house, ONE continuous slab and most likely the lowest costs with the highest benefit or least hassle.

And honestly - listen to your structural engineer, who will sign off on this and has liability insurance for his trade in case of emergency.

My 2 cents. Dirk Grafe

Hello Dirk

and thank you very much for your message. Why L-shaped stones should bend is not clear to me (as a layman), the stones have a kind of tongue-and-groove that interlock and are also frost-resistant (C75 concrete) and even load-bearing with 12 tons ("axle load") at the edge area. What else can still "twist" there?

"We don't need the space" is honestly meant at this point. We have already planned such a room for garden tools directly into the house/the basement and I honestly have no idea what we should do with 50 sqm (14 sqm in the house + 36 sqm under the garage) of space for even more garden tools. Our house already has over 25 sqm of storage areas besides the normal living spaces (without garage basement). A technical room is also added, so really plenty of space. That is intentionally planned this way. Another room is absolutely not needed.

Extending the floor has exactly the problem you mention. I think it’s called differently, but if the floor slab is extended (or the basement simply made larger), then another 36 sqm must be insulated to the same standard (KFW70). That is quite drastic in terms of costs and for a room that nobody needs. If the slab is simply left "as it is," the different dew points will probably destroy the foundation slab. So nothing gained. :-(

Nonetheless, many thanks from
Tabaluga

Hello,

We are currently solving it like shown in the photo. The stones are placed on a concrete strip
and will then be poured over. Inside, gravel is filled in and compacted. On top, we will then pave
and put a carport.

Maybe a solution for you too?

Regards

Joe

Hello,

The part is supposed to be between our house and the neighbor's garage, who built directly on the boundary (building encumbrance exists). Legally everything is clarified, we are also allowed to build up to the boundary and the neighbor also agrees to an extension of the building encumbrance, so that we can build slightly offset.

At what height did the neighbor build on the boundary? On unchanged ground level or how?

Rhenish greetings

Hello Dirk

and many thanks for your message. Why L-shaped stones should bend is not clear to me (as a layman), the stones have a kind of tongue and groove that interlock and are also frost-resistant (C75 concrete) and even in the edge area can withstand a load of 12 tons ("axle load"). What else could "twist" there?

The parts are not firmly connected to each other, so the frost resistance does you no good either, because water will work between the parts. If I still have photos, I will upload something showing what it looks like in case of damage - then you'll be convinced, promised.

Best regards
Dirk Grafe