As much as possible, yes, at least the main materials should be decided before you sign. That includes flooring type, tile ranges, countertop category, type of doors, and basic sanitary ware level. You don’t need to pick every tap model down to the last screw, but having a clear idea of quality and price range prevents a lot of drama.
When materials are not defined, contractors usually put “provisional sums” or very generic allowances in the contract. Later, when you visit actual showrooms, you might fall in love with items that are more expensive. That’s okay once or twice, but if it happens everywhere, your budget quietly explodes.
Finalising materials early also helps with timeline planning. Some items have long lead times; if you sign without checking availability, your project can get stuck waiting for one critical piece.
So, ideally: shortlist and roughly price the key materials, confirm that both you and the contractor understand what’s included, and then sign. It’s more homework at the start, but it saves arguments and surprises later, which is worth a lot.