I think i got stuck on this too. Just open a window then stand facing it.
You need to do like a crouching jump move so you can fit through the opened window.
im bout 35% through the game, i just started the 1st mission from thoses people (think there are 4 of em, when u are in the bar of the hotel)
on that 1 i can get into the garden, but i cnt get into the house, i can open 2 windows but cnt get through em. the door is also wont open.... iv tried 4 ages, ne1 know how to get passed this please?
yeah, i was stuck with that window for ages before i tried the jump crouch, it was halflife that reminded me to try it.