Lyric discussion by BaruchS 

Cover art for My Boy Builds Coffins lyrics by Florence + the Machine

While it may be, in some ways, literal, the way it's written up gives it a deeper meaning. This is a song about mortality. Coffins are a symbol of death, and this boy builds them for everyone. Kings and beggars both come to him because, in the end, no one escapes death. Everyone is mortal; the lines about how the boy has built one for him, the singer, and will build one for the listener supports this. The coffins aren't built for work or play because both work and play are, to some extent, optional. The coffins are a necessity; he HAS to build them because death doesn't stop. Every coffin is unique because every life is unique, but in the end everyone ends up dead and buried, and, just like it's a shame to through these carefully made coffins into the ground, it's a shame to see every unique life end and be buried.

Agreed. the boy exemplifies death in some ways.

This is the most spot on explanation I have read. Everyone else is either taking it too literally or far too abstractly. I feel that this is quite obviously the meaning of the song, whether or not the original idea came from a boyfriend. Basically it's about the impermanence of our existence.

This was the interpretation I had aswell.