I have a roommate who shares half my house equally. I claim his rent as income and have for the past two years. If I decide to put a small addition on my home to add a second bathroom, can I deduct this expenses since I am renting half my home. If I can, how much of the expense can I deduct in this years' taxes? 100% or 50%.

Rent income is reported on schedule E (Form 1040). Additions and improvements are capital expense. For rented portion, you MUST take depreciation every year.