Depends on the chassis.

Pure ADL Enviro 300s (ones with the ADL chassis) like those numbered 27xxx have side seats on the offside behind the cab and a large luggage pen over nearside front wheel. This only applies to the facelift version and not the original style.

MAN Enviro 300 (22xxx or 24xxx) and Scania Enviro 300 (28xxx) have forward facing single seats over each front wheelarch and small luggage pens behind them.

ADL Enviro 350H (Hybrid version of the ADL Enviro 300) (29xxx) have a forward facing single seat on the nearside front wheelarch and a pair of forward facing seats on the offside front wheelarch.

On the Volvo B7RLE chassis (Only ones that exsist are with Ulsterbus in Northern Ireland) are school spec and cram as many seats as possible. Over the nearside front wheel arch there is a pair facing forward and a pair facing the rear, whilst on the offside, there is a luggage pen then a pair of seats facing the rear.

Of course there is other modifications that come into effect such as extra luggage racks for airport work etc.