Xcode free space requirement

If you are saying you have 1GB of free memory then the problem is not what is grabbing that 1GB but the lack of memory in the first place.

This will get worse overtime because you are already hitting 0 bytes and operating system needs space to write out its files.

I strongly suggest you move essential files like photos a video media off to an external drive to free up some memory. These type of files being normally the largest and easiest to move. ( Do not delete any system files unless you know exactly what they are and removing them will not break you system)

Try and give yourself at least 10-15% free space as a minimum and keep at or above that.

There's a possibility that it's Time Machine local storage; though I'd use something like GrandPerspective to see what is taking up the most space.

If it is local backups...

Disable Time Machine Local Backup Storage

Here’s how to turn off local backups:

Launch Terminal from /Applications/Utilities
Enter the following command:
sudo tmutil disablelocal

Enter the admin password when requested to disable local backups

Re-Enable Local Time Machine Backups

sudo tmutil enablelocal

Remember, if you disable this feature you will not have local backups anymore, so if something goes wrong you will be out of luck. It’s important to have regular backups of your data, and so if you’re going to disable this be sure to still use Time Machine, perhaps even initiating a manual backup before you disconnect the drive, in order to preserve a recent copy of your data.

Source: http://osxdaily.com/2011/09/28/disable-time-machine-local-backups-in-mac-os-x-lion/

As regards temporary downloads…
They really ought to clean themselves up automatically, but if you're feeling brave & want to dig [very carefully] down into the filesystem, they live deep inside /private/var/folders/
I'd use something like EasyFind (freeware) to search for the string com.apple.appstore in Invisible files & folders, & look inside the most recently-modified folder matches (double click one of the found folders to open in Finder).
But I'd be really careful to be absolutely certain of what you're doing in there - I don't know if partial downloads will be obvious & in cleartext. I wouldn't touch anything with an unfathomable name.