Can Landlords Add Fines/Fees AFTER Lease Was Signed?

[Radley Estrada]

Hello Everyone,

I'm a landlord in Washington State and have a 4-plex with a shared secure entry door where tenants can buzz in their guests.  

I'm having a security issue with a tenant whose kids are going outside to play and not securely closing the door when they return.  Sometimes the door is left wide open late at night.  Other tenants have voiced their concerns regarding this issue.  I speak with ALL the tenants, and things get good for a few weeks - but then the issue of an open security door comes back up. 

In the interest of keeping my tenants safe, I'm in the process of installing security cameras.

If my security cameras catch the culprit(s) that are leaving the door open, can a landlord add/impose a fee if it was not part of their initial lease?

Any other landlords come across this issue?  And what was your remedy?

Thanks in advance everyone!