ᐅ Convert a single-family house (1965) into a semi-detached house and add a new adjoining semi-detached unit.

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 (996 sqm (10,722 sq ft)), here are some concrete questions regarding this:
Currently, on the right half of the plot stands a single-family house built in 1965, approximately 10x11m (33x36 ft) in size.
This single-family house is intended to be converted into one half of the semi-detached house due to setback regulations and plot size. This means that all windows on the side facing the center of the plot would be closed up, and this side would then serve as a party wall to the newly built semi-detached house.

There is no formal zoning plan; the project is based on the neighboring buildings, and the building authority is generally positive about the semi-detached house proposal.
Our questions are:

- Are there any particular points that need to be considered when attaching a newly built semi-detached house to an existing building from 1965? (Both would have basements)
Any no-gos or deal breakers?
- Are there specific issues to keep in mind when converting a single-family house into a semi-detached house?
- Since the single-family house has a footprint of about 10x11m (33x36 ft), would it be technically feasible to build the new semi-detached half with dimensions of approximately 10x14m (33x46 ft), assuming there are no building regulation restrictions and the roof pitch remains the same or similar?
The "extension" would concern the side under the two roof slopes, not the lengthwise side of the roof.
- Any other experiences or tips you might have regarding this project?

Or are there reasons why you would definitely not do this and would choose to build a detached single-family house on a separate plot instead?
The plot would be physically divided along the middle boundary of the semi-detached house.

Thank you very much!
Best regards

Freakyline schrieb:

- Are there any specific points to consider when attaching a newly built semi-detached house to an existing property from 1965? (Both would have a basement)
No-gos or showstoppers

- No shared party wall (each house would have its own external walls on all four sides)
- Constructing a basement for the second semi-detached house can be technically very challenging and costly – it can also quickly cause damage to the existing building
- But first: involve architects experienced in handling such challenges
- Will the land also be divided? Topics to consider include floor area ratio / plot ratio
- Are parking spaces possible?
- Internal access

Freakyline schrieb:

Since the detached house has a footprint of about 10x11m (33x36 feet), would a new semi-detached house with 10x14m (33x46 feet) be technically feasible?

Yes.

Freakyline schrieb:

Or are there reasons why you would definitely not do this and instead build a detached house on a separate plot? The property would be genuinely divided along the semi-detached house’s boundary.

A property from 1965 that also needs renovation? Why would you want to be tied to direct neighbors? Cost? I would only take on such an effort if the existing building is extremely affordable. There are many, sometimes expensive, renovation issues.

Thank you for the quick response.

Yes, the plot will be legally divided; the floor area ratio/site coverage ratio should be irrelevant since there is no development plan and based on the neighboring buildings, correct? At least, there is no visible limitation on the floor area ratio/site coverage ratio anywhere. Parking spaces are not a problem.

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

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

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Freakyline schrieb:

How can the basement of a newly built semi-detached house be more complex or expensive, and what factors does this depend on?

You are digging an excavation pit 3.50 m (11.5 ft) deep right next to an existing basement, where the basement slab might be at 2.70 m (8.9 ft). How do you think the existing structure will behave if it suddenly stands on a cliff edge—just like any house on a cliff when the soil slips and the house shifts?

Freakyline schrieb:

Floor area ratio / plot ratio limitation

The neighboring buildings could limit you.

Freakyline schrieb:

Yes, the plot will be actually divided

That is not always possible.

Freakyline schrieb:

We are from the Hamburg area

Which federal state? There is often a minimum plot size for properties. For example, if it is 650 m² (7000 sq ft), only a notional subdivision may be possible.

Freakyline schrieb:

The “extension” would concern the side beneath the two roof slopes, not the longitudinal side of the roof.

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