Questions about cost estimation

Hello,

I am currently working on an initial cost estimate of our current design and see one or two uncertainties:

- Cost estimation based on BRI: How is the roof calculated here? If I want to build a two-story house, it would be cheapest according to the cost estimate with full consideration of the roof volume to put a flat roof on it (which in reality is of course a more expensive variant than a normal gable roof). So is the roof volume left out and instead the unit value per m³ adjusted? For example, with a gable roof 340€/m³, with a flat roof 360€/m³ and then calculated without the roof volume.

- Cost estimation based on BGF: How is the roof calculated here? We are planning storage space under the roof, but this will virtually have no impact on the total costs. So do I simply take my BGF for the ground floor and upper floor and calculate my costs from that?

- Cost estimation based on WF: Are the "functional areas," such as corridors and stairs, included or only the areas of habitable rooms?

Which of the three variants is the best/most accurate?

Is it common practice with a partially developed basement to calculate using the corresponding factors for ground floor and upper floor and add the basement as a lump sum?

Our example:
BGF ground floor = 124m², upper floor = 117m² ==> 241m² * 1100 €/m² = 265,100€
Additional cost basement compared to slab = approx. 50,000€
+ double garage
+ outdoor facilities
+ incidental construction costs

Are we roughly off with this?

Many thanks and best regards
Holger

Sometimes Google is already helpful. Good etiquette also says that abbreviations should be written out at least once. In the attic, areas with a room height under 50 cm are not counted, areas up to 2 m at 50%, and all areas from 2 m clear room height are fully counted towards the living area.

With 1100 euros per sqm, you probably won't get very far either.

I have indeed tried to find an answer to my questions via Google, but I did not find a satisfactory and/or clear response to my questions there. Therefore, I also tried to explain exactly the points that are unclear to me. ==> In the gross cubic content (BRI), a changed design of the roof (e.g., flat roof vs. gable roof (vs. gable roof with a steeper pitch)) results in differently sized BRI despite possibly identical living or gross floor area (WF/BGF). In our case, the non-habitable roof in the design (max room height approx. 2m, purely storage space) accounts for about 20% of the total volume of the BRI. So if we were to design the roof as a flat roof, we could save 20%. And that cannot be correct.

Only the roof costs change, right? Material + labor hours for the floors below remain the same. No matter what the ratio of BRI roof to BRI living area is. It's not called price per cubic meter for nothing, but price per square meter.

So a flat roof costs nothing in the cost estimate because it has no volume, and a pitched roof with a floor area of 100m² costs, depending on the pitch, e.g. €26,800 (15°, €400/m³) or €100,000 (45°, €400/m³).

What is unclear to me is precisely the point of how the roof should be correctly considered in the estimate according to [BRI]. If the [BRI] for the entire building is only multiplied by one factor, this factor must depend on the type of roof and take into account the complexity/volume ratio. So, for example, with a flat roof, high complexity/low volume => higher cost factor per m³. And this is exactly what my initial question regarding [BRI] aims at.

The volume is only used to calculate the fee for the building application. Depending on the fee schedule, the difference between an assessable €30,000 and €100,000, 7x30 or 7x100, is a difference of €490 in the fee for the building application. (for M-V) Whereas in my building application, the cubic meter is set at €118 and not €400.

This would then be an assessable construction value of €8,850 compared to €29,500, resulting in a fee difference of €144.55. For a construction cost of €200,000, this is absolutely negligible.


Excerpt M-V

1
Building permit, preliminary decision

1.1
Approval of construction measures or structural installations, except advertising installations,
1.1.1
for each commenced €1,000 of assessable construction value 11 at least 60
1.1.2
in the simplified building permit procedure, also in case of tacit approval, for each commenced €1,000 of assessable construction value 7 at least 50