Do hollow-sounding walls need replastering, or can they be patched?

Hollow-sounding plaster is what a tradesperson calls 'blown' — it's lost its grip on the wall behind and is hanging on by not much. It doesn't automatically mean the whole wall is finished.
Do the tap test properly first. Work across the wall with a knuckle: sound plaster gives a dull, dead thud, while blown areas ring hollow and sometimes give slightly under your fingers. That tells you how much is actually loose — often it's a patch or two, not the lot.
Two things decide whether it can be patched or needs stripping back: size and cause. One or two isolated patches with the face intact and no sign of damp can usually be re-bonded — a plasterer injects adhesive behind the loose area, or cuts it back to sound plaster and skims over. That's a few hours' work, not a room's worth of upheaval.
Damp changes everything. If the hollow patch comes with a tide mark, staining or a musty smell, something is feeding moisture in — a leaking gutter, failed pointing, a bridged cavity or rising damp. Patch straight over that and it'll blow again within a couple of months, so the cause has to be found and fixed first or you'll pay twice.
Where blowing is widespread across a wall or ceiling, or old lime plaster is crumbling away, stripping back and reskimming the whole surface is usually better value than chasing patch after patch.
The tap test tells you how much has gone, not why — and the why is what matters. Get a plasterer to look before you commit. For a sense of the numbers, see our plastering cost guide, and when you're ready, get free quotes from vetted local plasterers — ask each to check for damp behind the blown area before quoting.