7 has occasional single deckers.
11 and 12 occaisonal double deckers.
And ditto for the 4.
Been following Bluestar on Bustimes for a few weeks: -
1 & 2 are near 100% double deck (single door with the odd dual door)
3 is mixed single/double.
4 is mixed single/double
5 mostly double - usually older Scanias (route is outside Southampton) but the odd single.
6 mostly double - allocation is from Lymington, so the odd Morebus liveried vehicle.
7 mixed
8 usually single deck
9 mixed
11 mostly single deck, some double.
12 mixed
16 mixed
17 Double deck - dual door - usually E400 '63 reg batch
18 Double deck - dual door - usually E400 'City' batch
PRH Double deck dual door - usually E400 '63 reg batch
QC Double deck - dual door - usually E400 'City' dedicated livery vehicle.
also one vehicle is used on X7 (Salisbury Red route)
The 35/36/39 + H1,H2, T1, T2 routes operated by e200 'classic or E200MMC.
Also the 'Unilink' routes which are all operated by liveried dual door E400MMC.
Southampton had a lot of funding for Euro 6 upgrades (but didn't impose a CAZ)so older E400 & newer Scania D/deck upgraded.
Older non euro 6 Scania Omnideckers (+ other vehicles) used on school routes and route 5 - which is outside Southampton.
That is the general pattern, there's inevitably odd exceptions.
As far as I can tell? there appear to be no height or length restrictions on any of the main routes?
Single door 'deckers are usually to be found on the out of town routes 1,2, 3, 4,9 12 but can be found at times on city routes.
Totton operate the 8, 9, 11, 12, 17, PRH, 35,36,39, H1, H2, T1, T2 & part of 4.
Eastleigh operate 1, 2, 3, 5, 7, 16, 18, QC, part 4, all Unilink + most of the school routes.
Lymington operate 6.