Even if it would only cost 100k... We would now (provided we could get craftsmen immediately) have to live on a construction site for at least 1 year...
A good carpentry company can raise your roof structure within 3 weeks, interior finishing and bathroom take another 3 months. Until then, you just stay in a holiday apartment, you can mentally book the 5000 euros for that under construction costs.
I don’t think anyone would add the 100k to the value of the place after renovation...
Of course. You yourself say that similar semi-detached houses like yours went unsanitary for 400k on less land. Currently you estimate 300k for yours.
Go to an architect or general contractor and say that you want to add a floor and name your budget for it. Take 5k for the holiday apartment as well as 10k for the garden demolition (that can also be done by Pawlow from the classifieds) and its redesign, deduct that beforehand.
Post the floor plans here, there are enough people here who can roughly calculate your budget for that.