In a dynamic spectrum sharing system, secondary users (SUs) attempt to maximise their throughput while preserving a given level of quality of service (QoS) for primary users (PUs). In doing so, the secondary network monitors the spectrum and decides on spectrum availability, which at times and for various reasons, may be erroneous. If spectrum is sensed as available, the secondary network selects an appropriate spectrum access strategy, and adjusts other communication parameters such as transmit power levels. In this study, the authors analyse primary service outage degradation caused by secondary access with non-ideal spectrum sensing. For different spectrum access strategies, namely, the overlay and the modified underlay spectrum access (MUSA), the authors derive a closed-form expression for the maximum outage probability degradation (MOD) in the presence of spectrum sensing errors and fading and show how adjustments by secondary service can result in attaining the desired QoS for PUs. Besides, for a given acceptable MOD in the primary network, the authors obtain a lower bound on the achievable expected rate of the secondary service in Nakagami-m and Rayleigh fading channels. Simulations confirm the analysis and show that MUSA considerably increases the secondary service rate, particularly when the activity of primary service is above 50%.