All advice / Planning and cost
Hidden usually means unspecified
Many surprise costs begin as an unclear assumption: what happens to demolition, waste, access, fittings, tile quantities, electrical work or the existing substrate?
Ask what the quote includes and what would be treated as a variation. Clarity at the start is more useful than trying to predict every possible condition.
Existing conditions can change the work
A bathroom is opened up during renovation, and the condition behind old finishes may affect the work required. The quote should explain how an unexpected condition is documented and priced.
Do not approve a change you do not understand. Ask for the reason, the cost and the effect on timing in writing.
Leave room for considered selections
Selections should match the room, the scope and the budget. Changing fittings or finishes late can affect ordering and the programme.
A measured room and fixed-scope written quote give you a stronger basis for deciding what to include before work begins.