It's really common to locate your leisure battery under the drivers seat in a lot of vehicles such as T5/6s. One of the questions we get asked a lot is what components we can fit under there with the battery.
The answer is usually you would only have room for the battery, split charge relay and 12V fuse box under a single seat.
With a double passenger seat, you would be able to fit the battery, split charge relay, 12V fuse box too. You should also have room for the mains consumer unit and battery charger as well.