If you have a "mother-in-law room" above your garage, you will need some way to keep it warm or cool when guests drop by. Since you do not need or want to warm or cool this room all the time, you need some options that work better, are less expensive, and more cost effective.
You Could Use Portable Heaters and Air Conditioners, But...
A portable heater consists of a small radiating heat unit that runs on gas or oil. It does produce plenty of heat to keep the room warm in colder months (e.g., when you have family staying over during Thanksgiving or Christmas). Unfortunately, these heaters need to be well-ventilated, or your guests could suffocate from the build-up of carbon monoxide. These heaters have also been known to start fires at random.
Portable air conditioners fare better because they can easily be vented through a window above the garage. The section of expandable ventilation hose also means that guests who stay in this room can move the air conditioner into different positions for personal comfort. The down side is that if you have guests stay for a few weeks or more, these air conditioners are really expensive when it comes to electricity.
The Better Choice: A Heating Pump
Heating pumps would be your best and most economical choice because they can change gears between heating and cooling with the flick of a switch. You are also heating and cooling a smaller space (i.e., one room versus an entire house). You also do not have to have the heat pump on all year long. Just turn it on to heat or cool a day before your guests are expected to arrive so that the room above the garage is the perfect temperature for them.
Installing a Heating Pump
Ask an HVAC contractor to install the heating pump in the room above your garage. Make sure that there is a way to keep dampness out of this area too. A plumber can help in this department by making suggestions about how you can keep the humidity out of this area. While the pump can control heat and cold, it cannot and will not control the humidity levels.
Servicing the Pump Annually
After you have a heat pump installed in the room above the garage, make sure you have it serviced annually. This makes sure that the pump is still functional when out-of-town guests drop by. You would not want your mother-in-law to drop in, and then stay a week in this room with no heat or no air conditioning!
Contact a company like All American Plumbing & Heating for more information and assistance.