Du kan ændre underforespørgslen til en lateral joinforbindelse:
select h.*, u.no_users
from houses h
cross join lateral (
select count(*) no_users
from users u
where u.house_id = h.house_id and u.status = 'active'
) u
where
u.cnt >= 100
and exists (
select 1
from custom_values cv
where cv.house_id = h.house_id and cv.type = 'mandatory' and lower(cv.name) = 'red'
)