That's the default under the Tenement Management Scheme. But there's a catch worth the whole guide.
The catch
The default only applies if your deeds are silent. Your title deeds can set a different rule — a higher threshold, a named factor, or conditions you have to meet first. Some deeds contain a “manager burden” that lets the original developer appoint the factor for a fixed initial period, during which owners can't simply vote them out. You cannot know your building's real number without checking your deeds. This is why “how many do we need?” is genuinely un-answerable in the abstract.
Assuming the standard rules apply, here's the detail
One vote per flat, regardless of flat size
A studio and a four-bed each get one vote.
Joint owners share one vote
If two people own a flat together and disagree, that vote usually isn't counted unless one owns more than half.
Majority means more than 50% of the votes allocated
An affirmative majority — which leads to the trap below.
Silence is not a yes
Owners who don't respond don't count as agreeing. You need people to actively vote for the change, so non-responders are your real obstacle — not the ones who vote no.
The 28-day window
Owners who didn't vote in favour have a limited period (28 days) to challenge the decision. Document everything.
The practical bottleneck
Finding people. In a real campaign, a huge share of the effort went into tracking down absentee landlords who don't live in the building and don't check the group chat. The public Scottish Landlord Register is how you find them — and landlords are often easier to win over than owner-occupiers, because a cheaper factor is pure margin to them.
The honest answer
So the honest answer to “how many owners need to agree” is: more than half — but which half, and under what rule, depends on your deeds.
This is information, not legal advice. Verify with a solicitor for your specific situation.