Can I double-up the roof joists in my single-storey rear extension to support a roof lantern?

There is undoubtedly a trend for single-storey rear extensions to incorporate a roof lantern nowadays. Whilst doubled-up or tripled-up joists will often suffice to support the roof lantern, it is dangerous to assume such a solution is adequate for your specific building without making the necessary loading calculations. To give yourself the peace of mind that the lantern has been adequately supported, you need to engage the services of a Structural Engineer.