It is a 3360 sq ft (living space) building.

Hello,I'm thinking about buying a property listed as 2 bedroom, but theres a ton of basement space and I want to convert it to a 4 bedroom.

To keep it cleanest, I would think your sibling should pay market rent for the separate business space and they get their share of the profits from the building as a whole per your agreement. 

I've already replaced the flooring and toilet in the upstairs bathroom.

so this means the appraisal for the refinance part of the brrrr may come in too low if i move forward with this plan.of course on the other hand the tiny bedroom is wasted space that nobody will occupy. prospective tenants who need 3 bedrooms will not apply and my time showing this unit to them will be a waste. i do plan to hold this house for at least 10 years. so i am leaning toward doing this in an effort to make it easier to rent and keep tenants.

I've been looking hard into that market space but would really like the advice of someone with property there. 

Hi Sam, I have had good experience working with Payless Floors located in Walpole and North Attleboro for carpet installation; good range of price and quality and their installation was on-time and done well.For doors and cabinets I would encourage you to look at the Habitat for Humanity Restore in North Attleboro.

We buffed the floors (rented the buffer at Home Depot and got a buffing 101 lesson on You Tube), painted the walls, got new socket plates, new appliances (fridge, dryer, washer).

How many empty spaces and what work do those lots need?

The property will be a rental that needs a good amount of work up front to fix it up (will be like a BRRRR) like electrical, plumbing, floors, paint, reno on bathroom and kitchen.