With 300,000 for the house, 140 sqm should be doable. We are currently building and have finished our calculations. Our rough additional costs: 15,000 for house connections completely, 10,000 costs for land acquisition (depending on the federal state), notary, building application, surveyor, KfW expert, etc., 40,000€ for all extras (kitchen, nicer front door, nicer bathroom parts, fireplace, etc.) excluding exterior work and 6,000€ for extra earthworks (this really depends on the property). One should by no means underestimate the enormous costs from extras. All suppliers of the construction company want to earn a lot extra. The fact that you have already ordered windows for 15,000 euros through the construction company doesn’t bother anyone (if you ordered the windows yourself, the following extras would definitely be included in the price to win you as a customer). Instead, there is then extra charging for the warm edge. If you don’t want burglars to pry open your patio door in 3 seconds, another 1,000€ will be added. And this goes across the board from trade to trade. We thought we had calculated well, but we underestimated some things. The planned 15,000 euros for the outdoor area have now already been calculated elsewhere and "used up". The carport will probably have to wait; the paving work will start next spring (move-in before Christmas). Again, do not underestimate the additional costs from extras. Or be immune to the "if you don’t buy it right now, you’ll regret it for the next 20 years" feeling. Because much of what you decide and install today lasts that long, or even longer (e.g. windows, stairs, doors // conditionally: kitchen, bathroom, expensive! flooring).