Are the search phase costs for house construction realistic?

We are currently still in the planning phase for the house construction. For this, I would like to know if the estimated costs are realistic because we have set our limit at a loan amount of €200,000. Are monthly maintenance costs of €150 realistic for a well-insulated Kfw70 house of about 130 sqm (heating, electricity, miscellaneous for a 2-person household)? We could afford €900 per month for the project.

Here is our own calculation for the house construction: Desired house about 130 sqm without basement, KfW 70 with gas central heating, ventilation, solar on the roof and average quality fittings

Turnkey house. €180,000
- Own work (insulation, floor coverings, painting, bathrooms, electrical) - €30,000
+ Plot 700 sqm incl. connections and notary. €35,000
+ Additional construction costs. €15,000
-------------------------------------------------------------------------------------------------
€200,000

For garden landscaping, furnishings (already have quite a bit), a little buffer and kitchen, we still had €30,000 equity.

Is the construction project realistic like this?

Are monthly maintenance costs of 150 € realistic for a well-insulated KFW70 house with about 130 sqm (heating, electricity, other expenses for a 2-person household)?

That is probably not enough. Among other things, you have to consider: electricity, gas, water and sewage (depending on the municipality + sealing fees), waste disposal, property tax, insurances). For safety, calculate with 250 euros.

So we were given a rough estimate of 2€/sqm for the additional costs, so for 150 sqm about 300,- €. Better to set it a bit too high than to miscalculate.