Charging a tenant after initial deposit estimate

[Chris Eaker]

My tenants who just moved out did quite a bit of damage to the property. I repaired everything that was damaged and sent them the letter listing all the damages and costs to repair and telling them they owed me an additional $800 over their deposit. NOW, I find out that the sewer drain serving two sinks is clogged. My contractor used a high pressure cleanout machine and blew out a bunch of junk. But the sinks still won't drain so there is something else big in there. He's going to have to bring out his camera and his 100-ft snake and attempt to unclog it. The tenants should be responsible for this cost, but I already sent them their deposit return/damages letter. Can I go back and add this now? If I need to go to court to collect the money they owe me, can I add this to the amount?