Civil engineering in self-performance / Supplementary amendment to the general contractor contract

Hello everyone,

we are building with a general contractor (GU). The construction service description includes the earthworks / civil engineering / foundation. However, after consulting with the site manager, we have removed this part and are carrying it out ourselves. A friend of my wife who is a civil engineer is available to us for this. Our plot only has a slight slope. We are building without a basement. So the work is manageable.

We have now received a credit of about 5,000 EUR. Omission of frost skirts including excavation, earthworks, topsoil removal, production of gravel cushion.

Instead of the frost skirts with clean layer, a gravel cushion with sub-base insulation for the foundation slab will now be used. This insulation was quoted at about 4,200 EUR. On average, we therefore only save about 800 EUR on earthworks!

The site manager said that it is still better this way, as the additional costs according to the construction service description are not yet included in the 5,000 EUR. And with the friend's civil engineer, we could immediately prepare the terrace, double garage, drainage, crane standing area, etc.

Unfortunately, we will never know in this way what we ultimately "saved" or overpaid. But what do you think: Is it okay to do it this way? According to the site manager, we would save at least the 20% markup from the general contractor. Is it wise to do without the frost skirts and instead equip the foundation slab with the sub-base insulation?

For a gravel bed, you should excavate and refill the entire area plus at least 1 meter around it to a depth of 80 cm.

If you have to replace the soil anyway or if the frost skirts need to be complexly shuttered, this can make sense. However, only the contractor can make a reliable statement. It is quite foggy in the crystal ball. A well-made bed is worthwhile, especially since the construction is significantly simpler with an under-insulated floor slab.

Overall, it is always worth using a local civil engineering company instead of the general contractor in the prefab house sector. This way, you won’t need a digger and a man as often later on, for small things like connecting drainage up to finished outdoor facilities.

Where the general contractor is now out of warranty claims in every case and could cause trouble with the floor slab

The GU is not exempt from any warranty . What is correct: the interface coordination must fit. However, the TE does not mention that the floor slab is also excluded from the GU (which is usually not a problem with timber prefabricated house builders). Therefore, only the compressive strength of the floor needs to be adequate, which can be properly proven. However, the insulation should always be included in the scope of services of the floor slab contractor.

The contractor [GU] is not exempt from any warranty . What is correct: the interface coordination must fit. However, the original poster [TE] does not mention that the slab is also taken out by the contractor (which is generally not a problem with timber prefabricated house builders). So only the compressive strength of the ground must be adequate, which can be properly demonstrated. However, the insulation should always be included in the scope of work of the slab constructor.

The slab with insulation is done by the contractor [GU]. We are supposed to demonstrate ground compaction / load-bearing capacity and the installed gravel bedding must be frost-resistant. This verification should be documented with transportation papers/manufacturer specifications.