Your rental contract may (and probably should) come with an after rate, which is what you would be charged if you extend. You could try asking at the rental counter when you pick up, but the rental agents are most likely instructed to say no extensions allowed and you must bring the car back. My suggestion would be to contact the local branch by phone sometime during your rental and read your contract through to see what the after rate may be if it is listed. Most likely, that extra day will be a signficatntly higher rate. You should probably have a back up plan if the extension is just not allowed.