Under Ca Civ. Code 1941.1, where certain conditions render your rental unit uninhabitable, you may hire a licensed professional for repairs and deduct your expenses from your monthly rent. You should consult with a landlord/tenant attorney in order to determine whether mold falls in this category.
Hope this helps.
Armen Taslakian. 818. 937.3590. www.TaslakianLaw.com. Note: Armen Taslakian is an attorney licensed in the State of California. The answer is provided for informational use only. One should not act or refrain to act solely based on the information provided. No attorney/client relationship is created unless an Agreement is signed by the attorney and the client.
Please see the following guide for California Landlords and Tenants. This guide outlines the rights and responsibilities for both the landlord and the tenant.
As the other attorney noted, the repair and deduct remedy is an option for the tenant. However, I always would advise you as the tenant to first contact your landlord and notify him/her of the defect. Hopefully the LL will repair the defect at no cost to you. If you have provided notice and the LL has failed to take any remedial action after more than enough time to do so has passed, then the repair and deduct remedy may be a more forceful and appropriate option to take.
If you do proceed with the deduct and repair remedy you should anticipate an eviction proceeding initiated against you by the LL for failure to pay your full rent. With this in mind it is a good idea for you to document all your communication in writing with the LL, to keep any and all estimates for repair and any and all receipts that can show what the repair work cost. Do not deduct from your rent more than what it cost to repair the defect. This will then be used by you to prove your case should there be a trial on the matter.
This response does not create an attorney-client relationship between you and I. I am not your lawyer and I am not representing you in the underlying issue stated in your question. The response I have offered is not intended to be relied upon, you should seek out an attorney to assist in this matter.