California Code, Civil Code - CIV § 1997.260

If a restriction on use of leased property by a tenant requires the landlord's consent for a change in use but provides no standard for giving or withholding consent, the restriction shall be construed to include an implied standard that the landlord's consent may not be unreasonably withheld. Whether the landlord's consent has been unreasonably withheld in a particular case is a question of fact on which the tenant has the burden of proof. The tenant may satisfy the burden of proof by showing that, in response to the tenant's written request for a statement of reasons for withholding consent, the landlord has failed, within a reasonable time, to state in writing a reasonable objection to the change in use.