The discount to detached exists mainly because a semi-detached buyer is trading a smaller footprint and a shared wall for a lower price point, which particularly suits a first-time buyer or a downsizer who wants less house and less yard without leaving the neighbourhood they know.
Age and legal status of any secondary suite matter as much as the semi-detached label itself. Two nearly identical units can price differently once permitted improvements, updated mechanicals or a legal suite are factored in, the same way they would on a detached home.
A number that actually reflects your specific unit needs its own comparables, not a citywide average. Get your hand-reviewed valuation from Joel Dyck.