Willowgrove's newer construction actually works in favour of that premium, since a secondary suite built or converted more recently is more likely to have the permit and inspection history that supports an income-approach valuation, unlike an older, undocumented conversion.
The buyer pool here also skews toward families moving up from a first home, which shapes how a two-family property gets marketed, since some buyers want the second unit for a tenant and others want it for a parent or adult child.
The exact number depends on that unit's condition and paperwork, not just the neighbourhood. Get your hand-reviewed valuation from Joel Dyck.