Scope, item by item
Every task, in the order it happens. Not "renovate bathroom" — the removal, the waterproofing, the tiling, the fitting, each priced.
What is excluded, in writing
The exclusions are the part that predicts your final bill. One with no exclusions is not necessarily wrong, but it leaves you nothing to check when something turns up that it did not mention.
What is being removed
Demolition and disposal are real cost. An estimate that does not mention them has either absorbed them or excluded them, and those are not the same thing.
What the price assumes about what nobody has seen
Every estimate assumes something about conditions behind a wall or under a floor. The useful question is whether it says so, and what happens to the price when the assumption turns out to be wrong.
Which parts need a licensed trade
Electrical, plumbing and gas work is priced differently depending on who carries it out — and on whether it is in the number at all.
Who supplies what
Materials responsibility decides who absorbs a delay when something arrives damaged. It belongs in writing before anything is ordered.
How a change is handled
A written change with its own price, agreed before the work happens. Anything else is a conversation you will be having about an invoice.