ᐅ Extension of a 1960s House and Energy-Efficient Renovation

Hello everyone,

The existing building was constructed in the 1960s. Over the years, various renovation works have been carried out (electrical system, windows, ground floor flooring, stairwell, etc.). However, the electric heating consumes a lot of energy. The rooms are small, and the current layout is no longer suitable for two households. On the north side, a garage extends along the entire length of the house on a half-level between the basement and the ground floor.

The idea:
After careful consideration, an extension of the existing building towards the southeast seems reasonable. This would result in a building footprint of 14x9m (existing 8x9m + extension 6x9m = 126 sqm (1356 sq ft)). Ultimately, two separate residential units should be created. A utility room for each apartment is essential. Additionally, it would be beneficial if the upper floor apartment could also use the attic as living space. This could probably be designed as an open plan, but I lack the imagination regarding the room layout. I also don’t know to what extent the exterior wall of the old building will pose challenges for planning. It will certainly have to remain in place.

The tricky part about the existing building is that you have to walk around the house to reach the entrance door on the west side. To avoid visitors having to pass the ground floor when coming to the upper floor, the entrance area should be directly accessible from the street (south side). The question is: what to do with the existing stairwell? Demolition is problematic because it provides access to the basement. Should the old entrance remain for the ground floor and only the upper floor get a new entrance from the south side? I am uncertain. I do not necessarily want to have a basement under the extension — mainly for cost reasons, and the existing basement is actually large enough.

Technical aspects:
I would like to replace the old electric heating with an air source heat pump combined with underfloor heating. The roof covering will soon need to be replaced as well. Since the roof will have to be redone due to the extension, this could be a good opportunity to consider adding a photovoltaic system. New windows will be installed everywhere, and the exterior facade needs insulation. Does it make sense to build the extension with built-in insulation from the start? What is the best way to insulate the old building?

What does the zoning plan say?
Type of use: "General residential area"
Site coverage ratio (floor area ratio) = 0.3
The maximum allowable footprint of buildings is 200 sqm (2153 sq ft) per detached house or 100 sqm (1076 sq ft) per semi-detached house (the calculation is based on the main building footprint, excluding terraces and balconies).
Floor area ratio = 0.6 (maximum 2 full stories)

For buildings with gable, hip, or pyramid roofs:
- Maximum ridge height: 10.5 m (34 ft)
- Maximum eaves height: 7 m (23 ft)

For buildings with flat or shed roofs:
- Maximum building height (top edge): 7.5 m (25 ft)

Roof pitch for gable/hip/pyramid roofs: 25–40 degrees
Roof pitch for shed roof: max. 10 degrees
Semi-detached houses and terraced or row-type houses must be built with uniform main ridge direction and consistent roof pitch.
Roof structures and recesses such as dormers, cross-gables, and loggias are permitted if their total width on each roof side does not exceed half of the width of the supporting exterior wall below. The width of cross-gables is limited to one third of the supporting wall width per roof side.
The height of dormers and cross-gables (measured from the attic floor level to the top of the dormer/cross-gable roof) and roof recesses may be up to 75% of the total roof height (measured from the attic floor level to the ridge).
Dormers and roof recesses must be set back at least 0.5 m (1.6 ft) from the outer edge of the exterior wall below.
Roof structures and recesses must maintain a minimum distance of 1.25 m (4.1 ft) from gables, ridges, and valleys, measured at the narrowest point.
Dormer surfaces must be designed as windows.

Now I hope to get some ideas. The project is still in the design phase. I am open to all suggestions.

Best regards
Sven