Of course you know that you could use low frequency signals (~100kHz etc)
over 20m with no problems (using miniature ferrite stick antennas on both
ends). The coding would remove any (?) noise problems. I have done remote
control with 60kHz using ferrite antennas and antenna/loop combinations in
the past and it works very well. It is trivial to achieve 2kHz or less
bandwidth at that frequency (FM), and this removes local noise almost
completely. My longest distance was 8m but the devices were
over-engineered and they would have worked at twice the distance. I do not
know if it is legal in the US though.

For a garage door opener I'd try infrared keyfob style tx and a normal IR
remote receiver with some simple optical enhancements to help the aiming.
At 20m a LED with 2 x 15 degrees beam opening will cover about 10x10m^2 of
target. With 2 inch plastic optics on a standard 0.04"^2 sensor the range
should be 10 times the range without optics.

Peter

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.