Mobile payment startup Square plans to grow from 400 employees to more than 1,000 by the end of next year. The company will move to San Francisco's mid-Market Street area to accommodate the growth. The new hires are expected to be predominately engineers and designers. Square will take a couple of floors at 1455 Market Street, less than a block from the headquarters Twitter moved into in June, according to media reports. It expects to move from the San Francisco Chronicle building at Fifth and Mission streets by mid-2013. The company, which makes credit card readers for smartphones and tablets, will begin processing international payments this year. It recently landed a deal to supply its payment services at 7,000 U.S. Starbucks outlets.