You can use a booster physically located half way, a kind of signal repeater, or you could try a directional antenna at the garages, that would leave you fee to move around at the house but have a stronger signal. I find explaining directional antennas is easiest by getting people to imagine the antennas are sticks rising from the ground and the strength of transmission is the length of a loop of string placed around them. If you make a circular loop with the stick in the middle, the string outlines the range to expect and it will be equal in all directions (omni-directional), if you pull the string to an oval you get more range in one axis and less in the other. If you pull it into a narrow loop with the stick at one end, it extends further in that direction but much less in the others. The area inside the loop, whatever its shape is where you get adequate coverage. The important thing to note is the narrower the loop the greater the range but unless you use a longer string (more power) you can't make the range greater no matter how tight you pull the loop. That explanation is rather simplified and doesn't take into account side lobes or terrain but it should give the general idea.
Brian.