Multigenerational house (two parties) on existing plot

Good evening everyone,

I have been considering this for quite some time now, but slowly I want to get more concrete. Logically, I haven't thought through everything yet, but I have an initial idea. You should also please think it through to see if, due to my lack of construction experience, I have any fundamental misconceptions or if I should already approach certain topics differently.

Regarding the situation:
I am fortunate that my family already owns a building plot of 650 sqm. This was developed about 20 years ago, is located at the end of a dead-end street (private road already paid for) and is practically just waiting to be built on. I have already obtained the development plan from the early 80s, which naturally imposes certain restrictions. To what extent exceptions are possible there is beyond my professional building knowledge—if they should even be necessary at all.

A few key data points: floor area ratio 0.4; plot ratio 0.5; one full storey; roof pitch 25-35 degrees; gable and hip roofs allowed; building height 3.50 m measured from ground floor floor level to top of rafter intersection with external wall roof section.

The plot is trapezoidal, about 30x20 meters. I can make a sketch if needed.

I would basically build alone, which should not be a financial problem given my income. An initial meeting with a mortgage advisor using my first naive estimated budget of around €500k did not raise any major concerns, especially since I have a fair amount of equity and the plot is priced quite high, which of course increases the overall value.

What is to be built:

[*]Two separate residential units—still open whether ground floor/upper floor separation or via a granny flat
[*]Approx. 200 sqm total living area, with approx. 120/80 sqm planned—especially at that size, having a granny flat is a bit tricky based on my previous research. Fundamentally 70 sqm is also fine, but it should not be less than that
[*]Either a basement or more living space in the larger apartment—I know this may sound strange but I really don’t mind where I have some extra space for a small gym, hobby room, etc.
[*]I have no preference for construction type—it does not have to last forever but should offer a high level of sound insulation, especially between the residential units—the environment is anything but noisy, but I always expect the worst
[*]Double garage for the first construction phase, for the second phase two carport spaces, etc., to be planned but not implemented immediately
[*]Outdoor area to be done by myself, residential unit turnkey

The basic idea is to fully complete the larger apartment, while the smaller unit is kept in reserve. To what extent completion is necessary for preserving the building fabric is beyond my knowledge. Definitely no bathroom, etc., will go in there—except of course for necessary connections.

Of course, I have taken some inspiration from the usual prefab house providers regarding possible implementations. I am not sure how desired concrete links, etc., are here.

My question now: Do I have a fundamental planning error with these data points at this stage? Is a budget of about €450,000 + 10% buffer (just for the first construction phase) completely unrealistic? I have, of course, read through the forum somewhat and seen prices per square meter, but I have no idea how these two construction phases influence the costs. Of course, I am aware that costs change drastically depending on the fittings, etc.—here it should be possible in planning to approach more reliable figures.

My approach now would be, after your plausibility check, to find a possible floor plan in the prefab house sector and then based on that evaluate other types of construction or discuss with the prefab house provider.

Thanks in advance :)

One hundred twenty square meters plus fitness room and double garage for you, and seventy square meters reserve for whom and when?

Building height 3.50m measured from ground floor floor level to top of rafter intersection exterior wall roof

Basically the eaves? So the exterior skin exterior wall?

Roof pitch 25-35 degrees;

120/80sqm

At 35 degrees there isn’t much living space left up there. With a knee wall of 70cm, in a 20 meter wide house there’s roughly 8 meters width of living space upstairs. That’s enough for fitness.
In a 10 meter wide house, just 3 meters, if I take a knee wall of 70...

: Legitimate question. The house will be located near the city on the outskirts of Stuttgart (in a broader sense). My parents currently live in a relatively large house in the countryside, and it is foreseeable that in about 15 years they will move into this apartment (barrier-free, local infrastructure, etc.).

Regarding the need for space, I have tentatively calculated for a two-person household in the large apartment, which for me means two rooms for each for "free" use, such as an office, etc. I have already worked a lot from home before and will work from there at least three days a week, so I value spatial separation and a quiet work environment. If I now also calculate with two potential children's rooms, there isn’t much left for "frivolities" like fitness. I realize that one of the capital mistakes is to create endless space in reserve. I am trying to keep it as reduced as possible, but my current 2.5-room new-build apartment with just under 65 sqm already shows me the limits of too little space despite a good layout...

: Thank you very much! Exactly in this topic, I currently lack spatial imagination or what these parameters actually mean in practice. Of course, I can understand the terms—but only by definition. The wording is also not entirely clear to me and comes as is from the zoning plan. Without fully understanding the effects in detail, I had already expected that I might have to start here first. At my building plot, three zoning plans intersect and about 150 meters away a fourth zoning plan. This means that the next building plot to the south of me is developed with 2.5 floors, the other two sides with my zoning plan and a former agricultural operation (house with two full floors). In the line of sight (150 meters from me), cubic houses with flat roofs and three full floors have already been erected.

I am not planning to build a high-rise there now, but currently a hip roof with two full floors would be my favorite. Does it make sense to get legal advice on building regulations here? Are the directly adjacent 2.5-story buildings an advantage (except that they are located on my south side...)? One of the houses was extended upwards about 15 years ago—but that falls under the other zoning plan. Incidentally, according to the plan my building plot falls into the village area (historically belonging to the agricultural operation), while the others are designated as general residential areas.

Before you start thinking about the appearance of the house, it would be sensible to design the room layout. The small apartment should therefore be barrier-reduced (Barrier-free is very precisely defined, quite difficult for a single-family house, and aims at easy handling with a wheelchair). But this would mean that this apartment should preferably be on the ground floor. This is where it gets interesting: do you really want to leave 80m2 unfinished on the ground floor with access to the garden, etc., empty for 15 years? The large apartment could then be divided over the ground floor/upper floor. Probably in the direction of a living kitchen on the ground floor (garden access) and living rooms, hobbies, etc. upstairs. And again the question – do I restrict myself in usage for 15 years? Or do I even like having the living areas on a different level than the "guest area" (kitchen, dining, terrace). After all, there are also people who love maisonette apartments. Once you have thought about the room concept for yourself, you can look at how it can be realized in which building envelope and what type of house fits with that and the development plan (with possible exceptions).

Also completely justified. You have basically anticipated the desired layout, but I see the problem regarding the size of the granny flat as critical only on the ground floor. This will probably be the first place where cuts are made to get the floor plan to fit, but I do not want a pure 2-room apartment as a granny flat. There should also be some quality of living there.

I am well aware that the vacancy is far from optimal. But I have to do something for the existing building plot as well ;-)

My current favorite envisions for apartment 1 on the ground floor a utility room, guest WC, kitchen, and living/dining area, with the remaining rooms on the first floor. Apartment 2 on the ground floor has its own utility room and two well-shaped rooms around the corner. It is important to me that storage options or a utility room are also provided for the second apartment. My parents believe that 50 sqm would be enough for them as well; I have a slightly different opinion, but I suppose a compromise can be reached, especially since I am the one holding back here. Since my father will likely soon have trouble climbing stairs, the ground floor is the only sensible option and also the point at which the existing house must be replaced.

May I link prefabricated houses here or post the floor plans? For those who want to google Massa Haus FamilyStyle 20.02 W. I am only interested in the exemplary layout – aside from details or conflicts with the development plan.

I am generally quite rational and approach building very unemotionally, as you might notice. Of course, I have my ideas, but all in all, I am a realist. Compromises are part of life.