Extension to old semi-detached house (60s)

Hello,
I am completely new here, have already read quite a bit and hope for your opinion to somewhat classify mine.
We are in our early 30s and have 2 small children (1, 3). Our apartment is effectively becoming too small and we are also gladly looking for something with a garden. So why not a house? We don’t really have any special requirements. 120m² of living space should be enough, but we have quite a bit of stuff.

We currently have access to a municipal plot of 600m², relatively narrow 13.8m in Erfurt. For a relaxed financing, I hoped for 400,000 total costs, but it will probably be tight with 165,000 for the plot. Normally, you don’t get anything in that location.

From the preliminary building inquiry, it appears that we will have to build onto the neighboring house so that a semi-detached house is created. The house is about 10m deep, has an asymmetrical roof, and a basement at ground level, the terrain is flat.

A standard house from one of the catalogs probably doesn’t really fit there. At least that’s my impression after the first phone calls. Now the choice remains, build with an architect or first draft from him and then general contractor/general planner.

Basically, it makes sense to build a basement as well, since otherwise the loads have to be absorbed and that’s money you can also put into the basement. I could imagine using a basement room as a work/hobby room. My wife, as a kindergarten teacher, has quite a lot of material for her work preparation. Where is such a basement insulated? The hobby room will be used rather rarely and does not have to be warm.

We were rather thinking of an open floor plan, although my wife insists that the noise from the kitchen and living room shouldn’t reach the bedrooms (especially the children’s). So rather hallway and a door to the open kitchen-living room? Basement stairs then open or preferably with a door because of the heat? I don’t even have a gut feeling about this.

Regarding the heating system, I am currently ambivalent, without information my tendency is towards a heat pump. Would an air heat pump be disturbing in a built-up area where no one else has an air heat pump? Is an annual performance factor over 3 even realistic for an air heat pump? With current electricity/gas prices, you would need an SPF 4 to be cost-neutral compared to gas.
Basically, it should be low-temperature underfloor heating, additional heating in the bathroom, e.g. infrared heater.

Building a house is still new territory for me and I hope to get some implementation ideas and warnings. What do you estimate the total cost (including additional costs...) for such a project to be?

Thank you very much for reading.

Best regards, Flo

Ring your potential neighbor and ask who they built with. Then I would call the company and ask about the costs for the semi-detached house.

Floor plans: I would simply have a look at the various manufacturers' websites. For example, Town & Country Mainz 128 is available with a length of 10 m. I would also visit the prefab house world to see model houses. Semi-detached houses are probably not available there, though. However, you can get a feel for the spaces, talk to potential general contractors, and align your tastes.

For the heat pump, please choose an air-to-water heat pump. A JVZ of 4 should be possible. Others here can help you with what to pay attention to.

Ring your potential neighbor and ask who they built with. Then I would call the company and ask about the costs for the semi-detached house.

According to the headline, the neighboring house is from the 60s. So calling the company probably won’t work... There are certainly many floor plans for semi-detached houses on the internet, but it depends on the spatial conditions: it starts with exactly how the dimensions of the neighbor’s house side are, to which you want to attach. Probably you won’t get around an individual planning here. The budget is not sufficient with the mentioned land price, and definitely not if there should be a basement. No way.

According to the headline, the neighboring house dates back to the 60s. So it probably won't help to call the company...

Oh, I had overlooked that.

Oh, what is special about the neighboring house is also that it has a full floor on the street side (south side), so only the ground floor and on the garden side 2 floors and the roof is shifted accordingly. That was a problem for Town & Country. What price would be more realistic? The basement was more meant for me that the additional costs are not much higher anymore. Regards flo

Hello Flo, you are describing a complex construction project with many unknowns. I am currently working on an identical construction project with a newly built extension to an existing building from the 1930s. Comprehensive professional advice is indispensable for laypeople at this stage. I am sure you have already received a contact from your region who could advise you professionally and free of charge. I have to agree with , the cost framework is very tight. The decision to build with or without a basement depends not only on financial feasibility but also significantly on the ground conditions. In my case, the foundation with a basement is only slightly more expensive than one with a strip foundation. In your project, securing evidence and the structural stability of the old building must also be taken into consideration.