Cost of a single-family house with a granny flat?

My estimate for the house without outdoor facilities but including ancillary construction costs: From €500,000

Depending on the equipment (floor coverings, heating, ventilation, roller shutters, smart home, etc.) of course significantly more.

Why do you want a granny flat? For renting out (which is usually pointless, even if it sounds good at first, especially if the budget is tight) or because of some compelling family situation or similar (then it may be unavoidable)? Do you already have the land or does it have to be included in the 300k? If you already have it, depending on the area, I can imagine that you can build a house, but significantly smaller. If that also has to be included and in your area the sqm of building land costs more than €12.50 (developed), it probably won't work.

For €1200 monthly, you can already get a loan of €400,000 with 2% repayment under good conditions. Provided you mean your €1200 as a pure installment without additional costs.

That is somewhat too general. The conditions also depend on the amount of equity available. I have €300k, a rate of about €1000/month, but with that I can't get a term of 25 years! However, I have the feeling that interest rates have dropped again since then (I closed in 11/18), maybe that will help. But for the house and the intended living space, the budget is definitely way too low.