You can probably do your own quick & dirty assessment by looking in Craigslist and checking the rental values in your neighborhood. Maybe visit one or two of them.
I do think adding a second bath would increase its rental value - much better for roommates. The extra office and usable back yard are certainly pluses and it would increase the homes over all appeal.
Please call me so we can arrange a time for me to meet with you and take a look at what you already have and give my opinion of how best to maximize your ROI.
It's pretty hard to give an estimate like that without seeing the property. I would contact an agency that handles rentals (we don't) and have them come take a look.
With a tenant in there make sure you do it with permits.
Lance King/Owner-Managing Broker