Why split
A long project with a single payment at the end concentrates risk on one side. Phased billing spreads it fairly.
Typical split
Deposit at booking, one or more phase invoices at defined boundaries, balance at handover. The boundaries are named in the scope register, not invented later.
What a phase boundary means
A defined, reviewable outcome — not "roughly halfway". If the outcome is not deliverable, the invoice waits.
Changing the schedule
If the project reshapes, the billing schedule is re-agreed in writing at the same time as the plan.