Costs for renovation and extension of a single-family house from the 70s

Hello everyone,

we want to renovate a bungalow built in 1971 with an unfinished attic and ideally have it extended upwards, and we hope for your collective knowledge about the approximate costs we can expect.

The bungalow measures 10x12m and has a full basement. The basement is also dry and has been partially used as a guest room and party room.
Currently, there is an oil heating system from the same year installed. This is to be replaced with a heat pump. For this, we would like to switch to underfloor heating. So either remove the screed and rebuild it or retrofit with dry screed and change the door heights. According to documents, the ground floor should be 2.70m high. However, to verify, we first need to remove the wooden ceiling.
The old wooden windows should be replaced with plastic windows and possibly 1-2 additional windows installed.
On the ground floor, some walls should be moved to create an open living/dining area. Preferably with an adjacent pantry and storage room. The ground floor should also have a guest WC with a shower and an office.
The electrical system and water pipes need to be renewed.
The currently open stairway to the basement should be closed off.

The wish is to extend the bungalow in timber frame construction with knee wall heights of 1.00-1.40m. Upstairs should fit three bedrooms, one with a dressing room, a bathroom, and a laundry/utility room.
Additionally, the house should be insulated and equipped with a controlled residential ventilation system. Whether decentralized or centralized does not matter to us, as we know both variants from friends and find both acceptable.
As own contributions, removing screed, wooden ceilings, and doors, renewing water pipes (support from family and already done ourselves), drywall installation, laying floors, and fitting doors are conceivable. Painting, of course, as well.

A photovoltaic system would also be desirable.

Currently, due to the favorable loan, a KFW85EE is desirable. The higher, the better, of course.

Thanks in advance
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Thanks in advance

What is your question?
Have you already been to the structural engineer regarding the load-bearing capacity?
Have you already been to the building authority concerning the approval issue?
What does the expert say about the costs of the potpourri of wishes? That already sounds almost close to new construction costs (minus the base slab and foundation walls).

That almost sounds close to new build costs (minus the base slab and foundation walls)

I thought so too. I am missing a site plan, north arrow, and a sectional drawing. I can hardly imagine a clear room height of 270 cm at the moment.

Requests for structural engineers and architects are underway. I just wanted to have a very rough cost estimate in advance. For example, heat pump 30k, underfloor heating 20k, interior finishing upper floor 30k, electrical work… etc. I lack the experience values here. I hope to find them here. The building authority says §34. The surrounding houses are almost all higher than our project. A carpenter says that the addition will roughly cost between 100-150k, depending on knee wall or full story, dormers, roof windows, etc. However, then we are standing on the bare reinforced concrete slab on the upper floor. The cutout for the stairs amounts to about 7k. The 275cm are marked, but there was screed etc. on top. According to documents, that is 9cm plus the floor. So already quite a bit is missing. However, I can only measure properly once the first boards of the wooden ceiling are removed. North is quite accurately „up“ on the plans

If I see that you are basically doing a major renovation + adding an extra floor, I would estimate the normal total costs at about half a million €. With your own work, it should be somewhat cheaper. What surprises me, however, is that with all the measures you only want to go for KfW85EE. Why not go directly for better efficiency? You are basically constructing a new building. I would simply choose a few centimeters more insulation and go straight for KfW40 or KfW55. Plus EE, of course. Price-wise, not such a big difference anymore.

If I see that you are basically doing a gut renovation + an extension, I would estimate the total normal costs to be about half a million euros. With your own work, it should be somewhat cheaper.
What surprises me, however, is that with all these measures you only want to aim for KfW85EE. Why not go directly for better efficiency? You are basically building a new house. I would simply choose a few centimeters more insulation and go directly for KfW40 or KfW55. Plus EE, of course. Not such a big difference in price anymore.

Hello Karsten,

thank you for the reply. KfW70EE or even KfW55EE was actually the goal. However, I assumed that there were further requirements. The conversation with an energy consultant is still pending.
If, in general, it's only a matter of a few centimeters more insulation, then the question doesn't even arise.