The LL is entitled to ask for 1 1/2 months rent as security. He is in violation of the rent security act to ask more. Depending on your agreement he may not change its terms until the lease expires.
A review of your lease terms should provide guidance as to whether your landlord can impose additional security deposit demands. As previously mentioned, a landlord can ask for one and a half month's rent as security. Whether your landlord can ask for more than you've already provided depends on your lease terms and depends on how much you have already given as security. Consult with an attorney to discuss the specifics of your case.
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my colleagues are correct, and I will only add:
Security deposits are to cover damages to the apartment/leased premises beyond normal wear/tear. Unless you drive a front-end loader, it's hard to see how your use of a parking space would cause damages. That's just a practical thought, there.
Most LL's charge the maximum in security deposit allowed by law (1 &1/2 month's rent), so he cannot ask for more than that. Also, your LL cannot ask for money that is at variance with the lease in the middle of a lease term; he may only ask for more money in a new lease.
The foregoing is not legal advice, and nothing in the foregoing shall be deemed to create an attorney client relationship. If you feel you need to speak with an attorney regarding your issue, it is recommended that you contact an attorney with expertise in your area of inquiry. The information related above is purely for informational purposes, and should not be acted upon without speaking with qualified counsel familiar with you specific situation and the laws related thereto.