Section 11-703. Presumptions and burden of proof

a. For the purpose of the
  proper administration of this chapter and to prevent evasion of the  tax
  hereby  imposed  it  shall  be  presumed  that  all premises are taxable
  premises and that all rent paid or required to be paid by  a  tenant  is
  base  rent  until the contrary is established, and the burden of proving
  that such presumptive base rent or any portion thereof is  not  included
  in  the  measure  of  the  tax  imposed  by this chapter shall be on the
  tenant.
    b. Where a tenant uses premises both for residential purposes  and  as
  taxable  premises and the tenant pays an undivided rent for the premises
  so used, it shall be conclusively presumed against such tenant that  the
  rent  ascribable  to  so  much  of  such  premises as is used as taxable
  premises shall be the amount which such tenant deducts as rent for  such
  premises  in  determining the tenant's federal income tax (as reduced by
  any disallowance of such deduction which is not being  contested)  which
  is fairly attributable to the tax period or tax year.