Financing plan -> Need for optimization?

House turnkey: €285,000

It is best if you clearly define for yourself what "turnkey" means in the offer. It may be that many extras are missing that you consider necessary. I do not know the offer. Therefore, hard to say.

Loan offer:
€400,000 with €60,000 equity -> €340,000
At 0.65% for 15 years with a monthly rate of €1000,
the remaining debt would be €185,000.

Offer already received? At which bank?

I'll shed some light on the matter:

Additional construction costs:
Soil survey
Provision interest
Earthworks
Disposal of sand (landfill)
Construction water
Construction electricity
Surveyor
Building application
Construction safety
Builder's insurance
Cleaning work
Construction supervisor

Calculate approximately 20,000 EUR

Interior finishing:
Floor coverings
Tiler
Top plaster (smooth)
Apply painter's fleece
Painting work ceiling/wall
Fitted kitchen

Calculate approximately 30,000 EUR material costs only (!)

Additional charge for window options, e.g., exterior foil
Additional charge for electrical, outdoor lighting
Additional charge for sanitary/heating, e.g., descaling system
Additional charge for tiles
Additional charge for interior doors/front door, e.g., door height at 2.13 m

I hope this helps for now...