I have renovated several houses in the past - the only Grade 2 being a barn conversion and I did, successfully put a second floor on it although the planning consent was a bit long-winded (digging out the foiundations to prove they'd ben underpinned). For the last eight years I've been renovating a 1847 station house on which I have put a 2-storey extension. The biggest problem was matching the brick - the originals came from a brickyard in the village, long since closed. I eventually found a localish company that made handmade bricks that were a close match but when they arrived they weren't an exact match as the originals were very soot-stained. Eventually, we discovered an easy answer - lay the new bricks with the reverse facing outward with the firing marks showing. Brilliant match - you can hardly tell the difference.

The point I'm making is that much is possible if you're inventive and take the planners along with you and make use of reclaimed materials where possible. It's never easy and certainly not cheap but it can be very satisfying. Best of luck!