That per the Washington State Landlord Tenant Act late fees must be “reasonable” and not punitive? The courts typically interpret this as being a maximum of $50 or 10% of one month’s rent, whichever is higher. The NWMLS lease actually indicates a maximum of 10% of one month’s rent. The safest bet is to take 10% of one month’s rent and divide that amount by 30 to come up with a daily amount. The courts have been known to disallow any late fee at all if the late fee is found to be punitive.