Approximate cost for 50 sqm extension

Hello dear forum participants,

I have been reading along for a while and have learned a lot here, really a great forum. Now things are finally getting more concrete for us and I would appreciate your assessment:

1. We are planning to buy a house around Berlin, which we would need to extend by about 50 sqm over two floors
2. The extension would be 35 sqm on the ground floor with 2 openings. Above that on the first floor, 15 sqm with one room and one opening, the rest roof terrace
3. The 35 sqm should be a living room and the 15 sqm a study, which means just bare rooms, no bathroom or similar
4. Everything should be turnkey and of good quality with parquet, lots of glass to the outside and underfloor heating (I know everyone has a different idea of what that means – let's say: a Golf, but no Porsche or used Dacia). The house has an A+ energy standard, so I would like to maintain that
4 The house purchase only makes sense for us if the extension is still within budget, so I am currently trying to estimate the costs with a +/-10% margin. I am also talking to architects in parallel.
5. How long would you say such a project takes approximately from planning to usability?
6. Of course, I also need a building permit

Is this enough for a rough estimate? From my research I get about €2200-3000 per sqm (all inclusive with architect etc.). So €110,000-150,000. Does that match your assessment?

We did something similar in 2021 / 2022. We added 50 sqm. However, only on the ground floor level. A roof terrace was created on the upper floor. For us, the extension became the living room and the pantry. However, we built with individual contracts and a lot of own work. We also renovated the entire ground floor. Costs for the shell construction including earthworks and connection to the existing building were just under 50k gross. Brick construction with concrete ceiling. Sealing ceiling and parapet 10k. Windows with shading 5k. Exterior plaster 8k. The interior construction is now hard to say, as, for example, screed and interior plaster were done throughout the entire ground floor. And electrical work done by ourselves, etc. But you see, just for plastering the sealed shell with windows we have already paid almost 75k and that only for one floor! From that perspective, I see 110k as turnkey as unfeasible.

I also see more the 150k + reserve 10% for unforeseen events.

Regarding the costs, I can only add that I see the 3000€/sqm. Although the bathrooms are included in a single-family house in the calculation, this should balance out with the breakthrough and the desire for extras: larger windows and roof terrace. Electrical installation and underfloor heating are also being installed. Can the heating handle 50sqm more in terms of capacity?

The house has A+ energy standard,

That sounds like a nearly new house. They usually have little buffer with the floor area ratio, so you build in the commuter belt on a small plot and fully utilize it. Is the house allowed to have an additional 50sqm or 40sqm of floor area?

Thank you for the answers! That already helps a lot.

Regarding [energiestandard]: the house is 20 years old with a geothermal heat pump.

With this answer, however, my mentioned problem with the property is not solved. Floor area ratio: ever heard of it?