Bungalow with basement for single-family house with 60m2 office, is it reasonable?

Own (presumably incorrect) cost estimate:

Option 1:
Bungalow: 115*2500 => 287,000 EUR
Basement: 115*750 => 86,250
Additional costs: => 74,650 (20%)
Total: 447,500 EUR

Option 2:
House: 150*2500 => 375,000 EUR
Base plate: => 20,000 EUR
Additional costs: => 79,000 (20%)
Total: 474,000 EUR

You will not be able to build the cellar with your 750€. The price will not be far from the living space price, as it is actually living space. It is within the thermal envelope, so it must be thoroughly insulated. Additionally, there is the heating. Also electric. You also need an appropriate room height, possibly windows for natural lighting. Larger earthworks than just a floor slab. If you read it like this, you will realize that with my cellar description I could have also described an upper floor.

If you want a cellar with living space quality, it costs the same as the living sqm, namely about €2500. But you don't want that: you want a proper cellar where there is an office in one corner. So you can take your proper cellar out of the thermal envelope and make one room a bit cozy in EL, unofficially creating a living space. An infrared heater could serve as heating. But honestly: who needs 115 sqm of cellar and wants to stay in a cellar room? You don't need that much storage space. You have to be able to afford planning a 60 sqm workshop. I always calculate €1000/sqm cellar costs, which would be €115,000 for workshop and cellar rooms. For this chunk of money, I would honestly build differently, call up Plan B, and/or rent a shed in the neighboring community for the years I need it. The stated 115 sqm living area is also rather limited in comfort, so personally, I would generate more living space rather than sink money into the cellar. If you put a cellar under 80 sqm floor area or give the cellar living space quality (if you need it and plan accordingly), the matter would look different again. Maybe also consider an outbuilding (shed), if allowed.

Does the proposal make sense or would a two-story single-family house with about 150m2 be more useful, especially for the plot?

To answer that, one would need to know your age, whether children play a role, how many people will live in the future house, if more will join.. Also: are the 500,000 only meant for the house including ancillary construction costs or should they also cover the land? Does it still have to be paid or why is a merely "adequate" sized house planned, which can't even include an office in its modest size? Of course, one weighs several options, but one shouldn't get too entangled with exotic considerations.

Keep in mind that with the bungalow you will get a correspondingly large area under the roof. We have a half-hipped roof and easily about 70sqm in the central corridor. It would be an idea to put your "office"/electrical workshop under the roof, insulate the roof there so you don't freeze, and then zone the roof into small sections. Then your electronics stay dry and you save yourself an expensive basement. You have enough storage space upstairs. A small staircase leading up, and that's it. For a technical room, 8sqm is enough for you.

Is the workshop used only occasionally as a hobby or also professionally? If it is just for hobby purposes and primarily involves electronic work, 60m2 is huge. Even 30m2 seems very much for that.

Possibly option 3 could be a two-story single-family house with a small office, no basement, and the workshop in the double garage. Which then, if possible, could maybe be built a little longer. Or the workshop could be attached as a separate room at the back of the garage.

Of course, it depends on how often the workshop is actually used and whether it really needs to have living space quality (heated).

Alternatively, as already mentioned, the workshop could be built separately as an outbuilding (shed), if possible.