Universities and colleges: the higher education trap
Further education is held at standard tier regardless of capacity. Higher education is not. A university venue expecting 800 or more people falls into the enhanced tier, with registration, documented measures and a security document.

The distinction that matters
Schedule 1 holds childcare, primary, secondary and further education at standard tier however many people are present. Higher education is deliberately not in that group.
So a sixth form college with a 1,000-capacity hall is standard tier, and a university with the same hall is enhanced tier. The buildings can be identical; the classification is not.
What that means for a university estate
A large estate will usually contain a mix: most buildings out of scope, many at standard tier, and a handful at enhanced tier: the big lecture theatres, the students' union, the sports arena, the graduation venue.
Enhanced tier premises need registration with the SIA, public protection measures kept under review, a security document, a designated senior individual, and re-provision of the document within 30 days of every revision.
Occupancy across a mixed estate
Assess each premises on its own realistic peak. A lecture theatre used at capacity in week one and half-empty in June is judged on week one.
Include staff, and include events. A venue that hosts graduation, open days or external conferences may cross a threshold on those days even if term-time use is well below it.
Enter your premises and expected occupancy. You get the tier for each site and the reasoning behind it. Free, no account.