Does Martyn's Law apply to churches and places of worship?
Yes, where 200 or more people may reasonably be expected present at the same time. But places of worship are held at standard tier by statute regardless of capacity, so even a cathedral holding thousands does not become enhanced tier on size alone.
In scope from 200
A place of worship is a qualifying use under Schedule 1. Where 200 or more individuals may reasonably be expected present at the same time, the premises is in scope and standard tier duties apply.
The figure includes everyone present — congregation, clergy, volunteers, choir, and anyone working on site.
Held at standard tier, whatever the capacity
This is the important part. Schedule 1 holds places of worship at standard tier regardless of how many people they hold. A church expecting 2,000 at a Christmas service is standard tier, not enhanced.
So the enhanced tier duties — registration, documented public protection measures, a security document and the 30 day re-submission clock — do not apply.
What that leaves you with
Notify the SIA, and have evacuation, invacuation, lockdown and communication procedures appropriate to your building.
For most places of worship that is a genuinely proportionate ask, and much of it will already exist in some form.
Halls and rooms let out to others
Where a hall attached to a place of worship is used mainly for something else — a nursery in the week, a hired function room at weekends — the use test may point elsewhere, and a different part of Schedule 1 may apply.
Consider how the premises is wholly or mainly used, not only what it is called.
Enter your premises and expected occupancy. You get the tier for each site and the reasoning behind it. Free, no account.