This is a little simpler then trying to draft a professional letter.
Im not sure what state you're in, but due to the extent of the issue, they may be a protected class, gor example hoarders are considered ill and can be a liability issue.
The best thing is to avoid any verbiage.
Just serve a 3 day curable notice, outlining the issue, simply, you can state current housekeeping not kept or abided per your lease agreement. Most lease agreements have a housekeeping clause.
To protect the asset.
In the notice there is a section to reference where it is noted in the lease.
Example;
Per lease agreement
Community Guidelines Addendum
Page 1, Bullet 7 and 9,
"Resident shall keep unit clean, sanitary and free of objectionable odors at all times"
"Resident shall ensure that trash other material can not accumulate to cause hazard or violation of health, fire and safety ordinances"
Sign and serve.
This way there is no liability on you due yo incorrect verbiage or letter etc.
Also, plan notices with no emotion shows severity of issue, Professional sounding letter leaves room for misinterpretation of the severity or timeliness the issue has to be rectified by.
Once the notice is served, more then likely resident will reach out. When this happens, simply state" I will be happy to discuss this further in detail in person, would you like to make an appointment to come in? Soonest appointment is last appointment of the day.. no sooner and not right there and then if resident walks in or approaches you in passing. Next day is always better
Do not discuss over the phone, in passing, or on property.
Having a sit down and require resident to come in, displays the severity of the issue and they will be more enticed to rectify sooner.
This will allow you time to gather your thoughts playout conversation in your head, especially if you know the resident, you should be able to predict their responses and steer the conversation where you want it to go.
Don't show ANY emotion, stick to lease agreement and what expectation is.. dont let them get you riled up. Let them know issue can be rectified and revisit issues with a 24 hour notice to enter to inspect housekeeping at least 2 days after 3 day notice expires.
And if the issue is not rectified.. serve "FINAL 3 day curable.. then continue with evict process if not rectified..after Final 3 Day Curable Notice.
With curable notices I always give two, first and final, this way when attending court, if they say "I never got notice" or " I didn't have sufficient time" goes out the window.
Hope this helps