Flat Roof/Veranda, Best Options

What are the best options for a flat roof?

I'm looking into building a flat roof onto the gable of my house. It's not visible from the road and I've plenty of space. he house is currently under construction and this is a last minute add on as I've been quoted 25k for a proper garage.

The idea is to build a car port at the gable. The flat roof will give me access to the attic (converted). I would then like to use the flat roof as a veranda. Due to height difference between the house and garden behind, it's possible to have a few steps from the roof straight into the garden.

So that's the project and I'm wondering what the best options are for a flat roof extension such as this. As the house has been built with timber frame and an eco friendly outlook I'm keen to continue as such.

My builder has put forward concrete or aliminium as an options for a roof.

Anyone with experience of this? Advice much appreciated?