ᐅ Multigenerational house (two households) on an existing plot of land

Good evening everyone,

I have been considering this for a while now, and I would like to start defining things more concretely. Naturally, I haven’t thought through every detail yet, but I do have a rough concept. I would appreciate it if you could review it as well to check whether, due to my lack of building experience, I might have some fundamental misunderstandings or if there are certain topics I should approach differently from the start.

Situation:
I am fortunate that my family already owns a building plot of 650 square meters (7000 square feet). It was developed about 20 years ago, is located at the end of a dead-end street (private road, already paid for), and is basically just waiting to be built on. I have obtained the zoning plan from the early 1980s, which naturally imposes certain restrictions. To what extent exceptions are possible is still beyond my construction-related knowledge — if they are necessary at all.

Some key data: floor area ratio 0.4; floor space index 0.5; one full story; roof pitch 25-35 degrees Celsius (77-95 Fahrenheit); gable and hip roofs permitted; building height 3.50 meters (11.5 feet), measured from the ground floor finish level to the top of the rafter at the outer wall at roof section.

The plot is trapezoidal, approximately 30 by 20 meters (100 by 65 feet). I can prepare a sketch if needed.

I would basically build on my own, which financially should not be a problem given my income. An initial consultation with a mortgage advisor based on my first, naively estimated budget of around €500,000 (about $545,000) did not raise any major concerns, especially since I have significant equity and the plot is quite expensive, which obviously increases the overall value.

What is to be built:

• Two separate residential units — it’s still open whether they will be divided between ground floor and upper floor or via a granny flat / secondary unit
• About 200 square meters (2,150 square feet) of total living space, with approximately 120/80 square meters (1,300/860 square feet) planned — based on my initial research, this size might make a secondary unit somewhat complicated. Basically, 70 square meters (750 square feet) would also be sufficient, but it should not fall below that
• Either a basement or a larger living area for the bigger unit — this might sound odd, but I don’t particularly care where I can have some extra space for a small gym, hobby room, etc.
• I am indifferent about the construction method — it doesn’t have to last forever but should offer a high standard of sound insulation, especially between the two units. The surroundings are far from noisy, but I always expect the worst-case scenario
• Double garage for the first construction phase; two carport spaces planned for the second phase, but not realized immediately
• Exterior landscaping will be done by myself; the residential unit delivered turnkey

The basic idea is to completely finish the larger unit to make it immediately habitable, while the smaller unit will be kept in reserve. I am not sure to what extent finishing is necessary there to maintain the building fabric. There definitely won’t be a bathroom, except for any required connections.

Of course, I have looked for inspiration among the usual prefab house providers for possible realizations. I am not sure how welcome direct references or links are here.

My question: with these key data, do I already have a fundamental planning mistake at this stage? Is a budget of about €450,000 (roughly $490,000) plus 10% contingency (for the first construction phase only) totally unrealistic? I have of course browsed through this forum seeing price per square meter figures, but I have no idea how these two construction phases would influence the costs. I understand costs vary drastically depending on fittings and finishes — but hopefully, during planning, one can get closer to more reliable values.

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

Thanks in advance 🙂