We had this in our old house and it was very useful, as we would be coming in and out the back by bicycle a lot.
No problems. I think it was about IR400 in old money to get installed.
some systems such the YALE one allow a key pad (as stated above) to be positioned on other parts of the house which may solve your problem.
should not be that expensive to be installed and will allow you to activate and de activate the alarm