Cost estimation for major renovation

To be on the safe side, I would calculate between 2300-2500 euros per square meter. It will be pretty tight for you

Yes, I think so too. With 2000€ renovation costs per square meter, it hardly seems feasible anymore.

We also don’t have a clear overview yet of which funding options are available. We could cover the purchase price from equity and planned to finance the majority of the renovation costs (about 350,000€). We just don’t know what that means as a monthly burden, since we don’t know exactly which funding at which interest rate is possible. All in all, the house should be paid off in 25 years, and we could afford around 1600€ monthly loan payments. The KFW offers much better conditions than a regular bank. But we don’t know exactly what would be possible for us under which conditions.

The attic counts as well, these are proper living spaces, so (minus the slight roof slopes) also with normal ceiling height. The neighboring house is a very large multi-family house, not identical in construction, which stands in a block perimeter development. It is estimated to have been built several years earlier, and the "semi-detached house" was then attached to the end of this multi-family house. The multi-family house is fully rented.

Back then, hardly anyone indulged in that much living space (except as a villa). Make sure that this total does not include rooms finished unofficially.

By the way, the windows are made of plastic and only 10 years old. I suspect they could also be left in place. However, they are insanely ugly and somehow disturb the overall appearance of the otherwise actually nice brick building.

If the windows were a single measure back then, this could indicate "measures only when absolutely necessary." Check whether there is a "concept" behind the previous modernization.

The house was used as a two-family house (one apartment on the ground floor, one on the upper floor). The attic is developed and there is also a permit for residential use of the attic. However, there is no kitchen and no bathroom installed in the attic. I strongly doubt that there was any concept for the installation of the windows... Everything else was not renovated either and is therefore in need of renovation.

The neighboring house is a very large multi-family house that is not identical in construction and is part of a perimeter block development. It is estimated to have been built a few years earlier, with the "semi-detached house" then attached to the end of this multi-family house. The multi-family house is fully rented out.

For me, that would be the red flag that makes any renovation cost estimate obsolete. :D Doesn’t that bother you at all? I’m sensitive about it and wouldn’t even dare to live in a semi-detached house. But to glue myself with a semi-detached house of unknown soundproofing quality (potentially none at all?) directly onto a fully rented multi-family house—I'd really be afraid of noise. In a semi-detached house, you only have one party who potentially still wants to live peacefully next to you for a long time, so consideration is usually given. In a multi-party building, people "only" rent and there are multiple parties, which increases the likelihood of having troublemakers and makes it simultaneously harder to identify the offending party. Maybe I’m paranoid because I’m sensitive and predisposed, but I just wanted to mention it. :)

That would be the red flag for me that makes any renovation cost estimate obsolete. :D Don’t you mind that at all? I’m sensitive about that and wouldn’t even dare to live in a semi-detached house. But to stick a semi-detached house with unknown soundproofing quality (potentially none at all?) right next to a fully rented apartment building, I’d really be afraid of noise. In a semi-detached house, you only have one party who potentially still wants to live in peace next to you for a long time, so usually consideration is shown. In an apartment building, people "only" rent and there are multiple parties, so the likelihood of having troublemakers increases, and at the same time it becomes harder to identify the offending party. Maybe I’m paranoid because I’m sensitive and have past issues, but I just wanted to mention it. :)

We live in the middle of Hamburg. So no, the attached apartment building doesn’t bother us at all. At the moment we live in a mid-terraced house from the 60s, before that always in apartment buildings. We have never had any problems.

The house was used as a two-family house (one apartment on the ground floor, one on the upper floor). The attic is developed and there is also a permit for residential purposes for the attic. However, no kitchen or bathroom is installed in the attic.

Based on the information and your situation, I would aim for the renovation to the KfW Efficiency House 85 EE (or 70 EE). 70 EE or better is probably difficult with "only" blown-in insulation. You can claim two residential units and thereby get a total loan for the eligible measures of 2 * 150k at a low interest rate plus a repayment grant. The repayment grant is then also 10% + 10% WPB at 85 EE (I assume, if the "new" windows don’t ruin that). For the energy certificate, look for an issuer on site and point out the funding with WPB. An energy demand certificate should definitely be bad enough – and a consumption certificate is probably not allowed at all. Since personal contribution is not possible, the mentioned €2000+ per m² is already realistic. You have to consider that your roof area is small in relation to the living space (2 floors + attic) and the facade should "only" receive blown-in insulation. These are two major items that partly drop out or then become cheaper.