Legal Question in Real Estate Law in California

The answer may depend upon whether the relative rights of the sharing parties are determined by their co-ownership of the garage, or some kind of an agreement short of co-ownership.

If the parties' rights to the garage are the result of their being its co-owners, either as joint tenants or as tenants in common, neither co-owner has any exclusive right to any square foot; the whole garage must be shared equally -- meaning, I suppose, whoever gets there first has the right to occupy whatever space he's occupying, even if it's 100% of the floor area. Messy, but that's the way co-ownership of real property works. Of course, the sometimes-chaotic results are frequently modified by formal or informal agreement between the co-owners. On occasion, however, co-owners of real property can't agree, and lawsuits for ouster and/or partition result. See Civil Code section 843.

If the sharing is the result of an agreement, including an agreement between co-owners of the garage, the amount and location of the square footage to which each party is entitled will depend upon interpretation of the agreement. Since there seems to be a dispute, I'd assume that the agreement is rather unspecific and/or the agreement is not in writing. One general rule often used by courts in handling lack of specificity in contracts is to try to determine the intent of the parties at the time of making the agreement. For example, back in 1988 Joe and Al began sharing the garage at the place where each had an apartment. Joe needed space for his 1956 Buick and a couple of trash barrels, while Al needed space for his motorcycle and a workbench and power tools. In litigation today, the court would probably consider the space needs of the original 1988 agreement (which needn't be in writing, or even oral, but can exist just based on the parties' conduct back then).

Hope this is somewhat helpful. If not, please re-ask with more details given.

Use a tape measure. Sorry. Just kidding. I agree with Mr Whipple that it is dependent on the documents that reflect your sharing relationship. In the absence of such documents, I suspect it will be presumed 50/50, but past history may be relevant to prove the parties agreement. The problems with joint tenancy are as stated, and are probably worst case.