What tactics should I use to increase the rent without loosing the tenant.

[Amer Swid]

So, I found a multiunit house and a single-family house both attached to each other, both owned by a church that put them up for sale. all units are already rented but lower than the market value. The single family house is rented for 1100 but supposed to be at 1300/month (my realtor told me so), and the other tenants in the multiunit property each pays 850 which supposed to be at 900/month. Another thing is that the least that they signed says that landlord will be responsible for trash, swere "basic fee" (not sure what that means) and lawn mowing. I really feels I need to increase the rent but the tenants are nice and clean and not sure how should I approach this. Their lease will be ending in 8 month from now.

[Ned J.]

Issue #1... the current lease wont end for 8 months. You are putting the cart before the horse. You cant do anything until the lease is up, so you are a good 5-6 months early to be worrying much about it. 

I'd take the 4-5 months to research the market and look at as many comps as you can. Pretend to be a tenant and look at all the rental sites to see what's out there, what's included in the rent and what's not. Make sure to compares apples to apples for the current condition of your unit. Only then will you get a real idea on what your unit will pull. 

Then its weight an increase and how much at one time vs the cost of turnover. You need to run the numbers and rough scenarios on how long it will take to rent, how much rehab etc etc. Turnover costs $$$...sometimes a lot of $$....and can eat that bump in rent for a LONG time.