Resident relatives are generally included
A personal umbrella typically covers you and family members living in your household, which in a multi-generational home can mean three generations under one policy. That is genuinely efficient compared with each adult carrying separate cover, and it is why the household structure is worth stating clearly at the quote.
Each driver still needs qualifying underlying cover
The umbrella sits above underlying policies. An adult child with their own auto policy at low limits can be inside the household for the umbrella and outside its requirements at the same time, which leaves a gap exactly where the household assumed there was none.
Residency is defined and it changes
An adult child who moves out, a relative who spends half the year elsewhere, a student at college. Each of those affects whether they are a covered household member. Nobody updates an umbrella when somebody moves, which is how cover is lost quietly.