Continue building after moving in?

I want to have a single-family house built (approx. 130m²) and have a fixed-price offer for various construction stages from a general contractor. Now I would like to finish "unimportant" rooms (basement, guest WC...) after moving in and, for example, lay tiles myself, etc. I think it would have to work like this: I would have to have the entire trade "tile laying" removed from the offer. What I want tiled before moving in, I would then have to tender myself. Am I thinking correctly, or is there another common practice? What is your experience: is the trade fairly calculated out price-wise, or do I end up paying more with such an action?

Thank you very much for your statements!

As a rule, no fair deduction is made because the construction contractor compensates for the lost business.

Such a clause is removed FROM the contract BEFORE the first signature, then you might be able to expect a fair credit.

You can also have individual rooms tiled, painted, etc., at some construction companies. A typical example: only the attic is prepared for finishing. Then the heating and screed might be installed there, and you do the rest yourself (only in the attic).