20th Century Fox has pre-emptively bought Mark Burton's spec "They Came From Upstairs." Sources peg the sale at $1.75 million against $2.25 million. The high-concept adventure tale is described as "Home Alone" meets "Gremlins." "Upstairs" originally went out as a pitch, drawing interest from several studios. But Burton, whose credits include "Madagascar" and "Wallace & Gromit: The Curse of the Were-Rabbit," pulled the project off the market, opting to write the screenplay himself. Barry Josephson is producing through his Fox-based shingle. Creative executive Bridget Humphrey championed the material and was instrumental in bringing it into Josephson Entertainment. Fox's Lisa Ellzey is shepherding "Upstairs" for the studio. Burton, who hails from the U.K., recently won BAFTA and Annie Awards for "Wallace & Gromit." He is repped by WMA, Berlin & Associates in London and attorney Joel McKuin.