I rent a 2 bed flat with my girlfriend (3 rooms inc combined living room and kitchen). The total rent is £1581. My girlfriend pays £650 and I pay £931 rent. The second bedroom I use exclusively as a home office. We agreed the rent split partly on the basis that I would be the only one using the office.
On what basis should I calculate the expense of my office?
The proportionate share of my rent - 1/3 of £931 = £310
The proportionate share of the total rent (the "real" value of the office) - 1/3 of £1581 = £527
The difference between mine and my girlfriend's rent (kind of arbitrary) - £281