Property acquisition requirement: green flat roof with only 400m2

Hello everyone.

We have been searching for a plot of land in the district of southern Wine Route for some time now. Unfortunately, we have not found one yet, as many private prices range between €400 to €600/m² – which is too expensive for just a plot of land.

Now, at the end of the year, due to a local allocation, there might be a chance to get a plot for €300/m². However, the plots are supposed to be only about 400m² in size or rather small.

The question for us is whether it is even possible to build a single-family house with ground floor + upper floor with about 150m² living space and still have enough space for a double garage, garden house, and garden.

Currently, we have a 120m² garden at our condominium. A small garden house fits on it, and there is still "a little" garden left. In general, it is quite small, though. However, we do not need much more.

For 150m² living space, one has to calculate a house footprint of about 90-100m² (depending on wall thickness, e.g., 11m wide x 9m deep) + 45m² for the garage.
The garage usually has to be 5m away from the street. The house must be 3m away from the property boundary.

If the garage is completely next to the house, the plot would have to be at least 20.5m wide, calculating 3m distance + 11m house + 6.5m garage.
You could save 2m if you turn the long side towards the garden, which would then be 18.5m.
In depth, the house would then extend 12 to 14 meters into the plot. The garage with 5m distance to the street and 7.5m length is about the same depth.

According to the calculation, you would have 20.5x12m or 18.5x14m of area up to the garden and thus 140m² to 150m² of garden area behind the house?

Is this rough calculation correct?

Also, as a building requirement because of (rain)water problems (clay soil and lack of space for large retention basins), it is stipulated that all houses must have a flat roof or possibly a slightly sloped roof with greenery. Whether this really helps much... no idea – but it will probably be the requirement, so one has to comply.

However, I have rarely (never) heard anything positive about flat roofs. Even in many new builds, I have always heard that water gets in somewhere or there are long-term problems with them.

What are your experiences with them?

I also have no idea yet about a roof that is greened but slightly sloped. Would that maybe be better for waterproofing?
What do the costs look like for such roofs – what about the effort over time?

Basically, the plot would be in the municipality where we currently live and are very satisfied. The location would therefore be top – 12 minutes / 6km to work.

The plots are supposed to be only about 400m² in size, i.e. rather small.

400 sqm is completely sufficient and in many regions of Germany even plots of this size are no longer affordable.

Here the question arises for us whether it is even possible to build a single-family house with ground floor + upper floor about 150m2 living space and still have enough space for a double garage, garden house and garden left.

Completely sufficient, as long as you don’t want to ride across your lands into the sunset like a cowboy.

With 150m2 living space, you have to reckon with a house footprint of about 90-100m2 (depending on wall thickness, e.g. 11 wide x 9 deep) + 45m2 for the garage.

a) With 2x75 sqm per floor you definitely stay in the two-digit range for built-up area. Why does a garage always have to be built? And then such a big one? Just leave out the garage and take parking spaces. Your car won’t be afraid in the open air at night. Instead, have a small garden shed for e.g. bicycles (or a bicycle garage) or garden tools.

So far, I have always heard, even with many new builds, that water gets in somewhere or that there are long-term problems with it.
What are your experiences with this?

A properly and skillfully built flat roof is just as good or bad as a normal pitched roof. No advantages or disadvantages (possibly even cheaper to build).

The plot size is always complaining on a high level. With clever planning, you will have a nice and quite large garden. Depending on the location of the plot, you can also integrate the front garden area into the garden area.

I already thanked you this afternoon for your response (or liked it).
Your response definitely helps that we would buy the building plot. I really hope that we get the chance to do so. Even though I find other roof shapes more visually appealing, I would not want to miss out on the location and generally the building plot itself because of that.

We want the garage because a marten, cats, etc. cannot get into the garage and additionally you get space for storage that is at least somewhat protected against theft.
The previous statement was about €1,000/m2 for the garage with a hipped roof. That would of course also be a lot of money for 6x7.5m. Before the plot is purchased, I can still consider using the money for other things.

The plots should, however, only be about 400m² in size or rather small. [...] Is this rough calculation correct?

Depending on the floor area ratio, I consider it quite possible that you’ll already be sweating a bit with the calculations for the garage + terrace + paths. Personally, I can't really warm up to modest houses with huge rooms for the metal kids, but after all, you’re not building my house.