How do I bathroom / kitchen / extension? etc.

100% recommend if only for cost plans at key points (ie: before planning, early/mid tender). The initial will ensure you are in the right ballpark whilst making a number of assumptions based on finishes/buildups that will be confirmed at a later stage. The 2nd will pick up on these along with any other scope creep items that will inevitably weave into the project.

A good cost plan should include itemised line items for each of the build stages (demolition, superstructure, internal finishes, fixtures and fittings, services et al, along with preliminaries and a contingency) so you can assess expensive elements and make judgement calls during the project rather than having a huge shock and needing to undo work once you get tender returns. It will be stripped out of costs so that tendering contractors can all price the same document and allow you to assess tender returns like-for-like. Ongoing, the document and associated return will be used for monthly valuations to best ensure you are only paying for works undertaken.