Project plan and financing for the expansion of the parental home?

Hello,

I have a question about project planning in connection with financing.

I want to expand my parents' house and need about 100K for that, which is well below the total value of the house and the land.

The whole thing is supposed to be financed normally through the bank. The mortgage will be registered in the land register.
We are very skilled in craftsmanship and do everything ourselves completely.

Is it necessary to hire an architect to create a project plan for this, or is it also possible without?
I am reluctant to spend a lot of money on it.

Thank you for your help.

So, the whole thing of course also depends on the scope of the expansion, but I would check with the building authority or something... What happens if there are any defects after the expansion? Does the insurance then pay?...