Living roof: high maintenance?

[siftingflour]

Dear All,
I am building an extension with a flat roof, two skylights.
My living room has floor to ceiling windows that look out over the garden... so my beautiful view will be a lot uglier now.
The roof is going to be zinc (because of high levels of lead theft round here) but the architect has suggested I have a living (sedum) roof instead.
It will look great, but I'm worried about maintenance
Do you have one?
Are they high maintenance?
Do they cost a lot more?
Advice very gratefully received.
Thanks!