The amount of improved cellular signal you get inside the building will depend on the signal strength outside (where ever you mount the outside antenna). Typically an in building repeater system will work for you if you can make a cell phone call from where-ever you might put the outside antenna. If you can make a call from outside of your vehicle or from the roof of your house or building, the signal is probably strong enough for the system to help you.
In Building Cellular Amplifier Sytems
A basic cellular repeater building amplifier system consists of an outside antenna to collect the signal from the cell tower, a cellular amplifier to boost the signal and an inside antenna to broadcast the improved signal inside the building. A basic system is easy to install with running cable being the hardest part.
You can put most phones into test mode and get a received signal strength reading. This is a number usually between -60db and over -100db. The closer to 50db, the better the signal. To make a reliable phone call you usually need a signal under -100db. A signal in the -70db range will give pretty good coverage inside the building depending on the system.
More Info on In Building Cellular Amplifier Systems.