ᐅ Semi-detached house in Hamburg with a general contractor on own land, two plus full attic floors, no basement

I’m looking for feedback on anything we might have overlooked. Tips on processes and organization are also helpful. (Details about the location, plot, and contractors involved will be shared later to keep things anonymous for now.)

We want to build a duplex and hire a general contractor (GC) for this. Each party has about €40,000 (approximately $44,000) available. The scope of self-performed work will differ slightly between the two parties but will mainly be limited to painting (including plastering and all related work) and flooring (excluding bathrooms). The attic may possibly be finished as self-performed work.

We have a few requirements:

• Each party: 2 adults (bedroom, office) + 3 children (individual bedrooms)
• Total house footprint around 165m² (approximately 1,775 sq ft). A preliminary building inquiry has already been made.
• Noise protection necessary due to aircraft noise.

We also have some preferences (order does not matter):

• Full attic living space
• kfw300 program
• Solid construction
• Brick facade
• Pitched roof
• Central ventilation system
• For both parties: rooms should not be too large (e.g., child’s room 10m² (110 sq ft), office 8m² (86 sq ft)); only one large open area for living and kitchen.
• Each half of the building should be easily separable in the future into an accessible unit (basement) and an apartment (upper floor + attic).

Full attic living space:
According to the plans, we may build two full stories. We want to retain as much garden space as possible and, for this reason among others, would like to add a fully functional attic. By “fully functional,” we mean it can be used as living and working space and forms its own separate part of the apartment—not an open area connected to the stairwell (for example, the parents’ bedroom would be here). We were advised that having a concrete floor between the upper floor and attic would be sensible to achieve this. The attic could also be raised with a knee wall (dormer wall), which would greatly improve the interior height. Since the plans don’t specify maximum ridge, eaves, or gable heights, it will likely be restricted only by required setback distances. The setback to the neighboring plot is currently set at 3 meters (about 10 feet) for driveway, bike storage, and garage as border construction. This leaves a good garden area on the other side of the house.

kfw300 program:
This is very helpful for financing, almost essential for us. However, it comes with certain requirements that may cause additional costs. Which parameters would you recommend adjusting here?

Current status:
We have discussed financing preliminarily and set a budget framework. We already own the land. We are currently in talks with several GCs (ranging from smaller architecture firms, medium-sized companies, to larger firms like Baudirekt). We plan to decide on a GC in about a month. The options each GC offers will understandably influence our decision. We also still need to clarify whether we truly need and can afford a fully finished attic or if a pitched roof attic that could be finished later would suffice. A basement would probably be more expensive and provide less livable space than a full attic, so it is no longer a serious option. We plan to consult two or three more GCs to get feedback on possibilities and pricing.

Please feel free to ask any questions! This is already a lot of help. Maybe there’s even some additional advice out there. Thanks!