Storage Space for tools and supplies

Two key words: Ordinary and Necessary. To the extent you're prepared to justify the Necessary square footage component, and that this is an Ordinary expense for your enterprise, then a Proportionate amount of expense for the otherwise vacant unit could be claimed. Since storage does Not require heat, plumbing, utilities, etc your credible deduction would be very limited, and possibly not worth pursuing ...

what i think Rainman12 is saying is that you can take depreciation   based on the square footage used to total square footage times number of days used to 365.   This assumes you're not already taking depreciation on the unit - such as might be the case if this was an apartment in an apartment building.    

@C-E-D  Having read through the responses from @Rainman12  and @12309  and generally agreeing , I would still want to know  (a) is this empty unit , purposely empty ?  i.e. is it not available for rent and is being used for  business as office + storage etc.  Or is this a case where you use  any un-rented unit as  office/storage. (b) is this  empty unit one of many at the same site?;  (c) is your depreciation based on the whole property  or based on each unit as a separate rental ?

To me if you just happen to have an empty unit and you are using it as storage / office till the unit is rented  i.e. it is available for rent all the time ( even when the  unit is being prepared for the next renter ), then  it is simpler to ignore  the fact that it is being used as storage  for tools etc.   If on the other hand this unit is not for rent and is used all the time as storage , and if the depreciation is on unit basis , then  business usage  and associated depreciation may be worth it.  But it may present an issue when you dispose of the property --- do you sell unit by unit  or the whole property and how do you allocate the gains/losses to the units.  The issue is similar to a multi-unit property with a club-house --- how do you allocate the proceeds from sale of units individually to the common usage unit.  Suggest  , if you have a multi-unit property, that you talk to a accounting person  and plan all this out ( because eventually you will dispose of the property and what you do now will impact that ) -- that is my two cents.