Assessment of incidental construction costs single-family house experiences

Good morning dear forum,

I have a question about our ancillary construction costs and would appreciate feedback/experiences. We are building a prefabricated house with a concrete slab on a 708m² plot. The planning is almost finalized, but the contract has not yet been signed. To estimate the ancillary construction costs, I have already had a discussion regarding the earthworks and received a first offer today. The plot needs to be filled, as it lies about 1.4 m deep. The concrete slab is 107.67 m² in size. An external storage room, the driveway, and the terrace will be added. However, these are not yet finally planned by the architect and are therefore only roughly estimated by the builder.

Here is the offer:
Statics for concrete slab €480
Remove topsoil and store to the side €1,450
Filling the terrain in the area of the concrete slab for the house, storage room, carport, driveway, and terrace with pit gravel (estimated 1,200 tons) €25,200
Dimpled membrane to neighboring wall as backfill protection €1,026
Drainage pipes for sewage and rainwater, estimated 70lfm €4,900
2 inspection shafts for sewage and rainwater €3,680
Insulation under concrete slab 120 mm €4,320
Concrete slab, d=25 cm €14,176
Net total €55,232
Gross total €65,726.08

Additional services gross
Gravel fillings backfill material €24.99 / ton
Empty conduits under concrete slab €34.75 / lfm

How do you estimate the costs?

Many thanks in advance and best regards
Chris

I have a question about our ancillary building costs and would appreciate feedback/experiences. We are building a prefabricated house on a 708m² plot with a slab-on-ground foundation. The planning is almost finalized, the work contract has not yet been signed. For estimating the ancillary building costs, I have already had a discussion regarding the earthworks and received a first offer today. The plot needs to be filled up, it is approximately 1.4 m deep. The slab-on-ground foundation is 107.67 m² in size. There is also an external storage room, the driveway, and the terrace. However, this is not yet finally planned by the architect and therefore only roughly estimated by the builder. [...] (1,200 tonnes estimated) €25,200 Noppenbahn as backfill protection against the neighbor's wall €1,026 Drainage pipes for wastewater and rainwater, 70 linear meters estimated €4,900

It is very unfortunate that you only discovered this forum so late, have a planning that is already "almost finalized," and still do not use it to discuss but only ask for second opinions on cost estimates. I read "work contract" in the singular as an indication of building with a general contractor (without a tender???), and the error in "roughly estimated by the builder" lies partly in "roughly estimated" but mainly in "by the builder." Replace "estimated" with "overestimated," and you will know what awaits you [hheh]. Exactly with these "we couldn't have known" positions, profit is made in the industry! Your architect—hopefully, but I would be surprised if without quotation marks—should have already "finally" planned at this point; except for textures, the package should long since be "closed."

How do you estimate the costs?

You can usually only compare that regionally with an offer from another. However, there are other costs added, so besides the ancillary construction costs (utilities, insurance, electricity, water, etc.) - keep that in mind and keep smiling!

What exactly do you mean by "1.4m deep"? How much needs to be filled in there?

Hello Christian,

I have a question about our incidental construction costs and would appreciate feedback / experiences.
How do you estimate the costs?

I would like to share my own experiences with you here. When it comes to incidental construction costs, there is a rule of thumb that you can expect about 10-15% of the construction costs. But as I said, it is a rule of thumb. I already find your list good, but I would like to add to it.

Is the plot of land already developed? In other words, are electricity, water, and telecom lines already at the property boundary? If not, additional costs will arise for you here. However, these depend on the municipality. If the connections exist, you still have to have them laid into the utility room (Hauswirtschaftsraum). This is often the responsibility of the builder. You are talking about 70m. Some costs will also come up there.

Have you already done a soil survey? That costs about €2000. Depending on the results, further costs may arise for the disposal of the soil (excavated soil for the foundation slab), which unfortunately cannot be calculated in advance. For example, if you have clay soil, clay is considered "contaminated." At the landfill, higher costs apply for disposal here.

Another point is construction electricity and construction water. Here it depends on your construction company what they will cover themselves.

Does this help you?

Is the plot already developed?

Well, the house planning is basically complete - you wouldn't even start that for an undeveloped plot. And the rule of thumb is way off, since the incidental building costs largely depend on the plot and not on the house.