D.C. Code § 47-904
Where no price or amount is paid or required to be paid for real property or where such price or amount is nominal, the consideration for the deed to such property, shall, for purposes of the tax imposed by this chapter, be construed to be the fair market value of the real property, and the tax shall be based upon such fair market value. In any such case, the return required to be filed with the deed shall contain such information as to the fair market value of the real property as the Mayor shall require. Whenever, in the opinion of the Mayor, a return does not contain sufficient information as to the fair market value of such real property, the Mayor is authorized to make a determination thereof from the best information available.
D.C. Code § 47-904