I've not found it a problem, lived/mortgaged a grade 2 listed building for the last 15 years. Buildings insurance can be a little problematic if trying to go with the big online discount insurers. I have buildings cover with my mortgage company, which was Woolwich now Barclay's, contents with M&S for it's bikey qualities mainly.
Live in it for a while before undertaking work, understand the building. The approach to extensions isn't about faking something new to match the existing anymore, more using quality local handmade materials, and producing something that is quality and will be judged as that over time.
If you're into it go for it, if you're trying to return a development profit – maybe think again.