ᐅ New construction of a two-family house with a separate apartment

We are planning to build a two-family house with a possible granny flat and a basement (watertight concrete shell) due to the proximity to the Rhine River in a new development area in Cologne.
Key data are a 7 m (23 ft) eaves height, which corresponds to two full floors, with a roof pitch of 40 degrees. The buildable area is 12 x 12 m (39 x 39 ft), on a plot of 570 sqm (18 x 31.5 m / 61 x 103 ft).
The ground floor is intended for the older generation, with 3-4 rooms, kitchen, hallway, and bathroom.
The upper floor is for 2 adults and 2 children, so it includes 2 children's rooms.
The attic or basement is planned to contain a small apartment for 1-2 people.
We are looking for ideas regarding the possible floor plan, construction companies, architects, and experiences with prefabricated house providers. Planned start is around the end of 2016. Any tips are greatly appreciated.

Hello,

charlypoll schrieb:

Planning in a new residential development in Cologne ....

Are 3 housing units permitted? Noise protection requirements? Budget?

Regards, Bauexperte

Yes, 3 residential units are permitted, with no requirements regarding soundproofing.
Budget (construction and additional costs only) approximately 250,000 € (euros).
No property transfer tax / real estate acquisition tax must be paid.

So, to me, the budget seems too low.

Saruss schrieb:
Well, to me the budget looks too low.

I feel the same. We built a two-story house plus a finished attic in an 11x11m (36x36 ft) footprint ourselves, but it is only a single-family home (which made the electrical, plumbing, and heating cheaper). At the planned price, one apartment might have to be dropped, or we must have built at a completely overpriced rate 🙁

How large did you imagine the house to be?

To me, the budget also seems clearly too low. After deducting additional construction costs, there might only be around 210-220 thousand left, which won’t be enough even without a basement, let alone with one.