Cost of extension to a single-family house - time span between building application and habitability?

Hello,
we are considering the purchase of a small semi-detached house (3 rooms, just over 60 sqm living space), for which we would have to build an extension.
Mainly, we (in the long term) need another room, and the living room is actually too small. The semi-detached house currently only has 3 rooms, one of which (which would have to serve as the only children's room) is located under the roof with poor insulation and a narrow low staircase. The 12 sqm bedroom is on the upper floor (already with beginning roof slopes) and the similarly small living room is on the ground floor. In the long term and with a planned 2nd child, it is definitely too small, so extension considerations.
Since, unfortunately, we cannot build towards the property boundary, only 2 other options come into question:

Option 1 would be to create about 36 sqm of living space on the ground floor, i.e. there would be 2 bedrooms (the current living room + 12 sqm new) and a large living room (20 - 24 sqm). On the upper floor there would still be another bedroom and under the roof a hobby/guest room.

Option 2 would be a half-side gable extension with about 12 - 15 sqm floor area up to the attic, in which the living room on the ground floor would be enlarged (estimated doubled), a second bedroom (12 - 15 sqm, as a walkthrough room, possibly later with its own staircase to the ground floor) would be created on the upper floor, and the attic chamber as the 2 children's room would possibly be somewhat enlarged.
Which option is more cost-effective and especially: what costs should one roughly expect for each?

With option 1, unfortunately, more land area would be lost, but one would not have to work on the roof structure. And the attic room would be freely available. Presumably costs less, but what price range should be planned? What should be calculated for the architect and other costs?

My partner is a trained mason (but has not been working in the profession for a long time), how much own work is realistic besides his working hours, how do you calculate the financial savings?

How long is the approximate time period between building permit application and habitability?
What costs could be assumed for a possible basement?

Questions upon questions... Thanks in advance!!!

What - no answer yet? I'll start. Brief and concise: Without a sketch, without pictures, without static calculations, without an idea, it's probably very difficult to make any statement here. Somehow this is very hard for the reader to grasp, impossible to calculate, because all the planning information is missing. Only to say that extensions usually cost more than you estimate yourself, because in addition to the pure costs for masonry, roof construction, floor slab, etc., there are the connections, the question whether the existing structure can even support it, etc. Reading through, the general question arises for me whether the property is the right one. Currently 60 m², planned extension 36 m², poor insulation, narrow stairs... Wouldn't it be more profitable to simply buy a larger property? Something like this is only really worthwhile if you get the semi-detached house almost for free. Maybe think about that again.

Thanks first of all for the answer. I have to try uploading the sketches. And the semi-detached house is unfortunately not a gift. It costs a hefty 200,000 € for 600 sqm of land and a very well-equipped house (tiles, corner bathtub and shower...). Unfortunately, there is little choice and high prices in our town. Building yourself would only be possible outside the town or in niches with tiny plots. The stairs are "only" narrow and steep to the attic room, the upper floor is okay. But precisely because of that, it is impractical for us as a children's room in the long run, so the house would only be an option for us with an extension. But for that, we somehow need a cost calculation to estimate the total financing. Only the roof is poorly insulated – the rest is in great condition. The owners only had guest rooms under the roof; it’s simply not designed for the daily use of a child. Living/usable area including basement is slightly over 90 sqm. And the 63 sqm of living space does not include the attic room (because of the sloping ceilings). Yes, I would definitely prefer to move into 20 sqm more and possibly extend it someday... As they always say: the location has to fit (it’s perfect), you can change almost everything about the house... To see how much money you need for that, I had hoped for “back-of-the-envelope calculations” like 1500 € per sqm or something. Unfortunately, I don’t have exact measurements of the house either, only the floor area and the exposé with sqm. Apparently, there are no building plans anymore (it’s from 1934)... But I will try making a sketch, until then I look forward to further opinions anyway.

Here is now a sketch in the hope that someone will take another look...

Hi, can you upload the files directly as PDFs?

Thank you very much!

Michael

The PDF file is too large, it does not upload. Can't you unzip the [Zip-Datei]? Sorry for the inconvenience...