This paper analyses the possibility to exploit opportunistic spectrum access (OSA) for short-range radio communication systems within indoor locations in dense urban areas. In particular, considering the service area of a primary system devoted to providing outdoor coverage in a dense urban scenario, the percentage of indoor locations where the secondary users can reuse the primary frequency band without disturbing the primary system or being disturbed is estimated. The analysis considers heterogeneous path loss models for the primary and secondary systems encompassing the characterization of outdoor, indoor and building penetration losses. Obtained results quantify how aspects like the location of the primary network elements and the considered interference margins to protect primary transmissions impact on the spatial availability of the primary band within the interior of the buildings.