Convert a single-family house (1965) into a semi-detached house and add a new semi-detached house

Hello everyone,

we are from the Hamburg area and currently have the following situation: We would like to move into a semi-detached house together with acquaintances. Since we are already in negotiations for a specific plot of land, we have the following concrete questions regarding this (996 sqm): Currently, there is a single-family house from 1965 on the right half of the plot, roughly measuring 10x11m. This single-family house should now be converted into one half of a semi-detached house for reasons of setback areas and plot size, i.e. all the windows on the house side facing the center of the plot would be closed and this side would then serve as the party wall to the newly built semi-detached house. There is no development plan, it is based on neighboring buildings, and the building authority is generally positive about the semi-detached house project. The following questions: - Are there points that must be specifically considered when adding a newly built semi-detached house to a building stock from 1965? (Both would have a basement) No-gos or showstoppers? - Are there special issues that must be considered when “converting” a single-family house into a semi-detached house? - Since the single-family house has a floor space of about 10x11m; would the newly built semi-detached house also be technically feasible with 10x14m, if there are no building regulations against it and only the roof pitch should be the same/similar? The “extension” would affect the side under the two roof slopes, not the longitudinal side of the roof. - Any other experiences, tips that you might have regarding the project? Or are there arguments why you would definitely not do this and instead build a single-family house on a separate plot? The plot would be actually divided at the semi-detached house center boundary. Many thanks to all of you! Best regards

- are there points that must be specially considered when building a new semi-detached house onto an existing building from 1965? (Both would have a basement)
NoGos or showstoppers

- no shared party wall (so each house has its own exterior walls on all four sides)
- basement for the second semi-detached house can be technically very challenging and will cost a lot of money - can also quickly cause damage to the existing building
- but first: get an architect on board who can handle such challenges
- will the plot also be divided? Topics include floor area ratio/site coverage ratio
- parking spaces possible?
- internal access

since the single-family house has a footprint of about 10x11m; would the new semi-detached house also be technically feasible with 10x14m

Yes.

Or are there arguments why you would definitely not do that and instead build a single-family house on a separate plot?
The plot would be really divided at the semi-detached house middle boundary.

A 1965 property that also needs to be renovated? Why tie yourself directly to neighbors? Costs? I would only go through such an effort if the existing building is extremely cheap. There are many, partly expensive, construction sites.

Thank you very much for the quick response.

Yes, the plot will be divided physically; the floor area ratio/plot ratio should be irrelevant, as it is according to neighboring development and without a development plan, right? At least, there is no visible floor area ratio/plot ratio limitation anywhere. Parking spaces are not a problem.

To what extent can the basement in the new semi-detached house be more elaborate/expensive, and what does that depend on?

If an existing building from 1965 still has so much space next to it, why not extend it and convert it into a two-family house (either way: show all floor plans, the section, and of course the plot).

To what extent can the basement of the newly built semi-detached house be more elaborate/expensive, or what does it depend on?

You dig a foundation pit 3.50m deep right next to an existing basement, where the basement slab might be at 2.70m. What do you think will happen to the existing structure if it simply stands on a cliff – like all houses on cliffs – when the soil slips and the house moves.

Floor area ratio/site coverage ratio limitation

The neighboring buildings could limit you.

Yes, the plot will be divided physically

That is not always possible.

we are from the Hamburg area

Which federal state. Often there is a minimum size for plots. If, for example, it were 650 sqm, only an ideal division would be possible.

The "extension" would affect the side under the two roof slopes, not the long side of the roof.

So the eaves side? Each would have its own roof.