The headline cost difference
A walk-in shower (low-profile tray, glass screen, mixer) typically adds £900-£1,800 to a standard bathroom refit. A wet room, a fully tanked floor with linear drain, no tray, no enclosure barrier, typically adds £2,800-£5,500 over a standard refit, and £4,000+ on a suspended timber floor.
The gap is the tanking: a wet room needs the entire wet zone (and ideally the whole floor) waterproofed with a membrane system, plus a slope built into the floor. A walk-in shower contains the water in a tray.
When wet rooms genuinely make sense
Wet rooms are the right answer for: very small bathrooms where a tray would dominate, accessible/level-access requirements, and design-led en-suites where the seamless look is the point. They're not the right answer just because you saw one on Instagram.
When a walk-in shower wins
Walk-ins are the right answer when: you want shower-only feel without the cost or commitment, you have a suspended timber floor (much less risk), and you may want to sell the house in the next 5 years (more buyer-friendly than full wet rooms in family homes).
Hidden costs people miss
Wet rooms need an extractor sized properly for the whole-room moisture load, usually a humidity-sensing in-line unit, £200-£400 fitted. They typically need re-tanking and re-tiling sooner than a walk-in (10-15 years vs 20+) because the floor sees daily soaking. Insurance: tell your insurer; some require notification.