Business income or house property income??

(1)  Property owned by a firm has to be treated as the property of the firm and not of its partners and its income can be assessed only in the hands of the firm - Bhai Sunder Dass & Sons v. CIT [1972] 85 ITR 28 (Delhi)/New Cotton & Wool Pressing Factory v. CIT [1967] 65 ITR 662 (Raj.)

(2) As renting of property is not supplimentary to the main object of business of the firm ( investment only in your case) so it can't be treated as business income.(pls ref. this case : Commissioner Of Income-Tax vs Arvindkumar Odhavji on 16 November, 1994)

so it would be treated as income from house property and not as income from business in the hands of firm.

A firms show its rental income from under income from House Property and claim the deduction U/s 24.