Hello Sci666,
you have already received many answers. Many have noted that 20 TEUR is right on the edge. Of course, it depends on your own standards. Example bathroom: My parents renovated their bathroom about 5 months ago, approximately 12 sqm, their cost estimate: 15 TEUR, in the end it was actually almost 30 TEUR. Of course, it wasn't done by themselves Friends renovated their bathroom on their own, estimated 8 TEUR, spent 10 TEUR. For my parents, it was due to the higher-quality fittings but also (!) unforeseen issues. Unfortunately, the screed had to be redone, which couldn't be seen before when the tiles were still laid. The friends forgot "small stuff" – glue, grout, etc. With that I want to express that as a layperson you simply cannot oversee everything. A small thing is forgotten here and a little something there, it adds up. Or unforeseen things like with my parents. That means to calculate generously, which you also wanted. But if everyone says it will be tight, that would make me suspicious.
Furthermore, NEVER in an old building WITHOUT an expert! That may cost about 2 TEU, but they can tell you what needs to be done and what doesn't and are liable for their mistakes. The walls could, for example, be damp, the roof urgently needs repair, etc. And that costs a lot!
Especially be cautious with bargains! Usually, there is a reason for that – the ones from the Volksbank are not from yesterday either The client wants their money and they want their commission. Do you know how long the property has already been on the market? That often already says something (but it depends on the current market situation – where are you building?).
BUT: Basically, your situation isn't that bad. =) The income is okay, only the equity is too low. Financing over 100% will be difficult and if it works, then at a very bad interest rate, which will cost you a lot of money in the long run. Saving equity for 1-2 years is more worthwhile.
Two points still: 1. It has already been mentioned – think about the follow-up financing! (rising interest rates, risk of no follow-up financing due to too low repayment and the corresponding lending value) 2. Think about loss of earnings if you want to have children