Extension of a residential house in bungalow style with a 30-degree hip roof

How do you know that the geothermal heat pump can easily heat an additional 170m²? Do you have that in writing or in the form of a calculation? Or did the installer say it orally back then, "Yeah yeah, you could also heat the attic with it"...

Honestly, I find it hard to believe that someone designs the expensive geothermal system with a factor >2.

I guess in the far east you are talking about hourly rates, not hourly wages, of 31 EUR net... otherwise, I would immediately become a craftsman there.

What you definitely need is a structural engineer who calculates whether your foundation even allows your project. There’s no point in discussing it further before that. Because, and this is very important to understand: engineers have to plan cost-effectively. No one thinks of giving a bungalow a foundation that would easily support a three-story building later on.

Why? Because the general contractor wants to save money... and because the client quickly asks stupid questions about why he should pay more for the foundation... "after all, you’re not building a three-story building, just a bungalow........." ;)

The expert report requires a foundation up to 1 m deep. So that the organic layers are completely penetrated up to 0.7 m.
I think as an alternative a soil replacement of at least 0.7 m would have had to take place. So 0.3 m is definitely too little.
That would also explain the cracks in the existing structure.

So if it was really founded like that, then that is already too little for the existing structure. An extension should not be done then...

Thank you for the hint.

As far as I remember, I still have to look for the photos, about 50 cm of soil was removed and filled and compacted with recycled concrete. Only there was gravel and the floor slab placed on top.
I had also documented the settlement cracks early and discussed them with the construction company.

The construction company is very reputable here and has built hundreds of solid houses. The price was about 20-30% above other providers at the time and we consciously decided for them. Therefore, misconduct would be extremely unusual. The problem is that we are in a mixed commercial area where, for example, 40-ton trucks drive directly past the house and cause vibrations. The neighbors all have similar problems.

I spoke with another construction company that recommended opening the soil outside at several corners of the house and checking the thickness of the floor slab and the substructure there. Then it can be reinforced if necessary.

How does that work? Is that a solution? Are there rough cost estimates?

How do you know that the geothermal heat pump can easily heat an additional 170m²? Do you have that in writing or in the form of a calculation? Or was it just verbally from the installer who said back then, "Yeah yeah, you could also heat the attic with it"...

Honestly, I find it hard to believe that someone would size the expensive geothermal system more than twice as large.

I assume in the far east you are talking about hourly rates, not hourly wages, of 31 EUR net... otherwise, I would immediately become a craftsman there.

What you definitely need is a structural engineer who calculates whether your foundation even supports your project. There’s no point in discussing further before that. Because, and this is very important to understand:
Engineers have to plan cost-effectively. No one would think of providing a bungalow with a foundation that can easily support a three-story building later.

Why? Because the general contractor wants to save money... and because the builder quickly asks silly questions about why they have to pay more for the foundation... "after all, you're not building a three-story building, just a bungalow........." ;)

I calculated the heat pump back then deliberately based on the heating load calculation and a maximum living area of about 270 sqm.
The heating load is calculated at just under 5.8, the heat pump is a 9 kW unit.
The conversion of the roof into usable/living space was always planned, but it was originally not supposed to be a fully functional living area.
However, since we want to keep our severely disabled daughter in the family home and not put her in a care facility, the attic is now to be converted into living space. It is not supposed to be an additional 170 sqm, but 100 sqm usable should be achieved.

Yes, of course, 31 € net refers to the hourly rate for a skilled worker in construction/trades.

Before the structural engineer, who ultimately only works with paper, I wanted to have the base slab and the ground checked. Only then would I commission the structural engineer.

Since we want to keep our severely disabled daughter in the residential building and not place her in an institution, the attic is now to be converted into living space. It should not be another 170 sqm, but 100 sqm usable space should be achievable.

If you like the current drive of the thread (discussion about sub-base and heating loads) so much, you can of course stick to this sparse information – but then please reduce the full quotes for the sake of other readers. Alternatively, you can follow my hints in post #8 and thereby realize that the professionals gathered here can provide suggestions for your extension project. Fait votre Jeux!

Sorry that I’m only getting around to writing something about this now, didn’t find the peace between the holidays ;)

So, with the additional 50 cm of RC crushed stone, the whole thing already sounds a bit more solid. In general, all organic soils under the house must be removed. With the additional 30 cm (08/16 or 00/32?), you have 80 cm and that is quite decent at first. If no strip/frost skirt has been installed, however, the recycled material must have certification as frost protection material. Otherwise, there will be damage due to frost heave.

The cushion is usually over-compacted (proven by plate load tests) and absorbs the initial settlements of the soil. Therefore, the settlement cracks make me somewhat suspicious.

Sandy soils are a good building ground at first, once the initial settlements have been completed, not much usually happens after that. Unless the groundwater level drops dramatically. Were dynamic probing tests also carried out during the geotechnical investigation? These reveal the density of the layers and whether there is still potential for settlement at depth.

Making any subsequent changes to the foundation is, in my opinion, very difficult. You can’t add more crushed stone because it cannot be compacted afterwards. Underpinning with lean concrete is also problematic because this must be done in a load-locking manner. That means concreting downward from the slab. The house must be excavated in sections for this, settlements cannot be ruled out, and the achieved bearing resistances are also poor due to the high water table...

In my opinion, ground improvement by an injection method, e.g., Uretek, is the only thing that might work. But I also wouldn’t want to have to pay for that o_O

Therefore, have a structural engineer calculate the whole thing and hope that it fits. Why cracks (really settlements?) have already occurred should be checked again.

Points in favor, in my opinion:
- Are the organic soils completely excavated?
- Was the sand base properly recompacted before the crushed stone was installed? A plate load test is also necessary here (a drop plate is sufficient)
- Check what was installed (certification frost protection) and in what thickness
- Have compaction proofs for the crushed stone cushion shown

Approach the earthworks contractor and have them show the proofs

Oh, and Happy New Year!

Sorry that I’m only getting around to writing something about this now, didn’t find peace during the holidays ;)

So, with the additional 50 cm of RC gravel the whole thing already sounds a bit more solid. In general, all organic soils under the house must be removed. With the additional 30 cm (08/16 or 00/32?), you have 80 cm, which is quite decent to start with. If no strip/frost apron is installed, the recycled material must have certification as frost protection material. Otherwise, damage from frost heave can occur.

The cushion is usually overcompacted (proven by plate load tests) and absorbs the initial settlements of the soil. That’s why the settlement cracks make me somewhat puzzled.

Sandy soils are basically a good subsoil; once the initial settlements are complete, not much usually happens anymore. Except if the groundwater level drops dramatically. Were dynamic probing tests also performed during the geotechnical investigation? From those, you can identify the density of the layers and whether there is still potential for settlements at depth.

Making changes to the foundation afterwards is, in my opinion, very difficult. You can’t add more gravel because it can’t be compacted properly. Underpinning with lean concrete is also problematic because this must be done monolithically. This means concreting from the base slab downward. The house must be excavated section by section, settlements cannot be ruled out, and the achieved bearing capacities are mediocre due to the high water table...

In my opinion, soil improvement by injection methods such as Uretek is the only thing that could work. But even that I wouldn’t want to have to pay for o_O

Therefore, have a structural engineer calculate the whole thing and hope that it fits as is. Why cracks (really from settlements?) have already appeared should be checked again.

Points in favor, in my opinion, are:
-have all the organic soils been completely excavated?
-was the sand subgrade properly recompacted before the gravel was installed? A plate load test is necessary here as well (a drop plate is sufficient)
-check what was installed (frost protection certification) and in what thickness
-ask to see compaction evidence for the gravel cushion

Approach the earthworks contractor and ask to see the evidence

Oh, and Happy New Year!

Likewise, a healthy New Year.

Thank you very much for the really helpful tips and the detailed explanations. As a layperson, that’s exactly the kind of thing I hoped to find in this forum. Now I know what I need to do to move forward.

It’s a pity that many other forum users either assume a lot of expert knowledge or just drop a one-liner.