The Men's coat

I'm keen to experiment (operative word!) with making a topcoat, I guess something in the style of a covert coat but not using covert cloth.

Is there a straightforward way to adapt the coat pattern for this purpose? I'm aware that a proper overcoat will have loads of differences – I'm really just talking about lengthening and presumably not having so much waist suppression.

Things that have come to mind are lengthening to the knee, no sidebody, raised top button. Should the extra room be taken out of all the other seams? Feels like it would make sense as I'd presumably want a bit of space everywhere. Not that I'd imagine this would go over more than a jumper.

Thanks all!

Robbie