a story where they can't walk more than a few steps from eachother, set maybe between season 4 and 5 summer....i think it's willows fault. i think they stay at buffys place...

I can't remember why/how but they have to be within 6ft of each other in this-
Denial by The Enemy of Reality.
Forced to be close together for an infinite amount of time, it stands to reason that mortal enemies would drive each other insane. But when it comes to a self-righteous, uptight slayer and an irritating, chipped vampire, there is no reason.

In Favours by Pale Rider (on this site) they can't be more than a few feet away from each other because they are bonded.

I remember this one.... willow did some kind of spell... I think it was either to keep spike in Giles apartment or out of.... and then she went out of town somewhere. The spell messed up and kept spike and Buffy bound together so they couldn't be very far apart. Eventually willow popped back to remove the spell but at that point spike and Buffy had already gotten very friendly
I THINK spike got upset with Buffy over something after that and went back to Giles.... Buffy followed and they made up and at the end I THINK everyone walked in on them having sex. Wasn't a very long story I think 30k words or less