Two-family house, optimize?

, that looks like Waldfeld

that looks like Waldfeld

Just quickly copied from the city's side

if I have a plot size of 750m², I am allowed to build on 1/3 of it, in my case 250m² the current plan has a built-up area of 175m², what else would need to be included?

it is the wish and the decision of the person

It is really nice of you that you want to plan a granny flat for your child. But the whole thing also has to be affordable and reasonably feasible, so I am of the opinion that there are also limits somewhere to fulfilling the child's wishes. Or does he/she want to pay for the granny flat themselves? At 18 years old, you normally also don’t know where life will take you and how long you will stay in one place. Even with rental income, a granny flat usually does not pay off and very few people have enough cash to just pay for it like that. You will probably need more credit for it = less equity share = higher interest rate.

cool sausage - as an 18-year-old I wish for an apartment and get one built. But if it’s financially feasible, then go ahead.
Although I would also like to note here: your budget will be tight for the wishes.

The question about the granny flat keeps coming up here because many have the mistaken idea that you build a nice granny flat, put a solvent tenant in it, and automatically finance your house that way. Which of course doesn’t work like that, but this idea keeps popping up. You have clarified that.

But that also means: it doesn’t have to be a granny flat in the classical sense, with all the requirements you have if you want to officially register it as a granny flat (btw: sometimes this is also regulated by the development plan – meaning: it could be that such a thing is not allowed. So inquire about that too).
But here it is supposed to be a place for a young man (did I read that correctly?) who belongs to the family. Am I right in assuming that the other three children are half-siblings of the said young man? How do they get along with each other?

Kbt is right: the slope is not included in the planning, but with such a slope it definitely should be. It makes no sense to build a house that is planned for a flat building plot.

Here: because of the slope, a living area on another level could be very easily created for the adult child. It does not have to be a “real” granny flat, so it can be connected to the rest of the house via an open corridor and the rest of that floor can serve as storage space for the main family. Given the requirements of the development plan, the solution jumps out at you: “living area adult child in the basement plus storage space in the rear basement area for the main family.”
But it can also be the other way round: the adult son gets the attic floor. Advantage: kitchen etc. could move to the basement for the main family and you have direct garden access. There are two solutions, some want the kitchen, living, dining room on street level because you don’t have to carry your stuff from the car far into the kitchen and accept that you cannot go directly from the kitchen to the garden (the worse option for barbecue evenings but better for groceries), or kitchen, dining, living downstairs, direct garden access, but the shopping bag has to be carried down the stairs. (on a downhill plot).
You have to think about what is more important to you. My favorite would be the direct access to the garden, I’d carry the half pig over the stairs once in a while *g*.
Do you have a downhill- or uphill-facing plot?

Coming back to my earlier comment: How do the children get along with each other? One is already 15 years old, so the process of becoming independent won’t take that long anymore. Would it be an option to plan the older child as a kind of “appendage” to the living area of the adult child? So maybe plan a “level for children who have grown out of their parents’ home”? Would they possibly share the sanitary area? Or is that absolutely not an option?

That’s all something to weigh up.

But basically: with such a slope the construction becomes even more expensive. You won’t manage that with a proper EWL and the further wishes (sauna etc.) within your budget. Everyone will have to make some compromises.

Therefore: one floor for the adult or soon-to-be adult children, shared bathroom and the eldest gets two rooms, one bedroom, one living area with kitchen corner. The not-quite-grown children still live with the Ellis but sleep in the separate “[ErwachsenenKinderBereich]”. That already saves money. And you have to. Just the earthworks for the plot will probably amount to six-figures...

what else would need to be included?

Garages, parking spaces, driveways, terraces, paths – although there may still be adjustments here (exceeding the floor area ratio). You will definitely have three to four parking spaces/garages including driveways as well as one terrace. Double garage 6x8m = 48 sqm Two parking spaces 5x5m = 25 sqm Terrace flat rate 50 sqm Garden house flat rate 20 sqm Driveways flat rate 75 sqm Then you would already have 393 sqm built upon – that adds up quickly. If, for example, you only have a floor area ratio of 0.2 at your place, you would be allowed to build 150 sqm – with ancillary areas 225 sqm. And your house plan will collapse completely like that. With a floor area ratio of 0.4, it is 300 sqm, with ancillary areas 450 sqm – you won’t be far off if you still have to build the driveways more generously. § 19 Floor Area Ratio, Permissible Floor Area (1) The floor area ratio indicates how many square meters of floor area per square meter of land area are permitted in accordance with paragraph 3. (2) Permissible floor area is the portion of the building plot calculated according to paragraph 1 that may be covered by buildings. (3) For determining the permissible floor area, the area of the building plot that lies within the building land and behind the street boundary line set in the development plan is decisive. If no street boundary line is set, the area of the building plot behind the actual street boundary or the area designated in the development plan as decisive for determining the permissible floor area is decisive. (4) When determining the floor area, the floor areas of 1. garages and parking spaces with their driveways, 2. ancillary facilities within the meaning of § 14, 3. building structures below the ground surface that only underbuild the building plot, must be included. The permissible floor area may be exceeded by up to 50 percent by the floor areas of the facilities mentioned in sentence 1, but not more than up to a floor area ratio of 0.8; further minor exceedances may be permitted. The development plan may contain provisions deviating from sentence 2. As long as the development plan does not stipulate otherwise, exemptions from compliance with the limits resulting from sentence 2 may be allowed in individual cases 1. for exceedances with minor effects on the natural functions of the soil or 2. if compliance with the limits would lead to a significant impairment of appropriate land use.