Yes it is mate. It's 1 button & I can assure you no one else, in the office or otherwise gives a flying...donut.
But by all means take to any tailor, or your mummy and get them to sew on another button.
Of course - then people may notice that you had one button, but now have two and start running a book behind your back betting on when you will add another button. Or maybe they'll all start wearing one button jackets and then you'll be wearing the only two button jacket in the office. Don't want to worry you or anything but I've heard next year the three button jacket is going to be the 'must-have' item for the sophisticated office.