I think 90driver is spot on; if you can put your router (or a repeater) in a good location in/near/by the house (line of sight to your barn), and the receiver in an appropriate position at the barn (i.e. line of sight to the house) then you have a good chance.
you need to avoid substantial obstructions between them (like masonry walls, large metal objects, or dense woodland).
I haven't tested the limits of my wifi, but my digital cordless BT phone (quoted range 50m indoors, 300m outdoors) works fine at 500m from home (the furthest I can get without something getting in the way).
another thing you can try if your web-cam (or router) has a separate antenna; (and this does work with wifi - used to do this to borrow the pub's wifi when I lived on a boat) is to put your antenna in a 'dish' (I used an old wok) and point it in the right direction.