Originally Posted by MichaelW

The limiting factor will be the tyre clearance at the brake and/or the frame. You may be able to fit 28mm but note that there is a lot of variation in actual width and wider variants may not fit.
If you have fender eyelets, you need about 5mm of space to fit them so may not be able to use 28mm.

If you wanted to fit knobbly tyres [and fenders] you probably bought the wrong bike. You don't need nobbles in the rain, just a tough, puncture resistant commuter/training grade tyre. Dont skimp on tyre quality, this is the no.1 cause of being late for work.