Domestic quality is ok for infrequent use, trade will be more solid underfoot and heavier.
Double will be lighter and longer when stored, triple heavier, more solid underfoot but easier to store.
Do you need to reach the roof apex from the ground?
Measure floor-to-ceiling on each floor add these plus loft height plus 1m. You’ll need to add extra for below floor height too, e.g. 200mm joists, etc.
If you only need eaves height then leave the loft height out.
The extra 1m gives you something to hold on to
A bit of Pythagoras to get the diagonal length. 8m vertical height needs 2m away at the bottom (1:4) = 8.25m
Or you could drop a tape measure out of an upstairs window (hold one end!) and add a bit 😀
Essential accessories for me:
Rope to tie the ladder securely
Ladder leveller for the bottom (we have a sloping plot)
Ladder standoff for eaves (don’t rest it on gutters and window sills, etc)
This leaflet