D.C. Mun. Regs. tit. 9, r. 9-124
Net rents and royalties from real property located in the District shall be allocable to the District.
Net rents and royalties from tangible personal property shall be allocable to the District, as follows:
The extent of utilization of tangible personal property in the District shall be determined by multiplying the rents and royalties by a fraction, the numerator of which is the number of days of physical location of the property in the District during the rental or royalty period in the taxable year and the denominator of which is the number of days of physical location of the property everywhere during all rental or royalty periods in the taxable year.
If the physical location of the property during the rental or royalty period is unknown or unascertainable by the taxpayer, the tangible personal property is utilized in the State in which the property was located at the time the rental or royalty payer obtained possession.
Gains and profits and losses from sales or exchanges of real property located in the District, including capital assets, shall be allocable to the District.
Gains and profits and losses from sales or exchanges of tangible personal property, including capital assets, by a taxpayer engaged in trade or business in the District shall be allocable to the District in the following instances:
Gains and profits and losses from sales or exchanges of intangible personal property, including capital assets, are allocable to the District if the taxpayer's commercial domicile is in the District.
Interest and dividends from District sources are allocable to the District, except interest and dividends specifically excluded from tax under § 1 of Title 10 of the Act.
Rents and royalties from patents, copyrights, trademarks, service marks, secret processes and formulas, goodwill, franchises, and other like property are allocable to the District, as follows:
A patent is utilized in a State to the extent that it is employed in production, fabrication, manufacturing, or other processing in the State or to the extent a patented product is produced in the State.
A copyright is utilized in a State to the extent that printing or other publication originates in the State.
Income from sales of tangible personal property to the United States by corporations and unincorporated businesses is from a District source and is allocable to the District unless the entity's principal place of business is located outside the District, and the property is delivered from outside the District for use outside the District.
All other non-business income which is derived from sources within the District shall be allocable to the District.
If income is allocable within and outside the District under this section, the expenses, losses, and other deductions arising from production of that income shall be similarly allocable.
Losses incurred in any transaction entered into for the production of non-business income shall be allowed only to the extent that any profits from the transaction would be taxable under the Act.
D.C. Mun. Regs. tit. 9, r. 9-124