ᐅ Planning a single-family house (basement + ground floor + upper floor) on a 480 m² plot

Hello! In another thread, I was advised to present my project here. First of all, we are still at the very beginning of the planning process, have luckily secured a plot of land, and now need to study everything carefully. Tips, criticism, suggestions, etc., are always welcome.

Here are "our" key data:

- We are 2 adults, no children, and none planned for the time being
- Electricity consumption: about 3,000 - 3,500 kWh
- Hot water consumption: I don’t have exact data right now, but it’s rather high because we shower with warm water every day
- Fairly handy, so we would like to do at least the painting ourselves (interior + exterior), lay floor coverings (except tiles), and do the garden completely ourselves (we already have experience in all these areas)

Here are the key data for the plot:

- Plot size 480 m² (5,167 ft²)
- New development area
- 2 parking spaces required
- 2 full floors mandatory according to the development plan / zoning plan (building permit / planning permission)
- Ridge direction East-West, renewable energy preferred according to the development plan, photovoltaic as an independent roofing layer is possible

And here is what we want for the house:

- Ground floor with living room, kitchen, guest room, and bathroom with shower
- Upper floor with bedroom, walk-in closet, bathroom with shower, office 1, office 2
- Basement with technical room, utility room, possibly sauna and fitness area, but this might shift to just fitness + sauna in the garden
- Central long straight staircase as a design element
- About 80 m² (860 ft²) living space per floor
- Garage: We want an attached (prefabricated?) garage with direct access to the house plus a carport on the opposite side of the house (unfortunately required by the development/zoning plan)

According to the current plan (but I am still very uncertain here, so criticism is always welcome):

- Build according to KfW55 standard, even if there is no subsidy, or at least close to it—KfW40 does not seem worthwhile to me
- Natural gas connection is already laid to the plot, so my current preference is natural gas heating combined with solar thermal energy (for hot water and heating)
- Photovoltaics are currently not planned, as I don’t see the advantage given the low feed-in tariff
- A ventilation system, although I’m not sure yet if it should be central or decentralized

I can’t think of anything else at the moment—comments on anything are always welcome.

Thanks and best regards!
Elias_dee