Assessment of the financial framework for house construction

You have already achieved a good savings rate for quite some time, equity is available. Land prices seem okay, roughly calculate 500m² x 200 € plus 150m² house at 2000 € per m². Then add incidental construction costs of 50,000 €. That gives you an order of magnitude. Installment max. 1/3 of household income, with total housing and incidental costs preferably staying under 40%. Should be feasible over 30 years, with an installment of under 1500 € per month and 3% repayment.