Our non-listed, detached cottage was built 400+ years ago. It used to be a small mill but was converted to a residence during the Victorian era. At that point lots of the original character was lost. Until 1999 it was owned by the mansion on who's land it stood and was lived in by estate workers.
Apart from occasional maintenance I don't think much was done back then with any thought for the fabric of the building as the (Victorian) front door for example had so many layers of paint it was untrue! At the front the lovely old windows were replaced by ugly, single glazed 1960s type. Also, at some stage, the original stone roof was removed, sold and replaced with slates....grrr! After the cottage was sold off in 1999, the new owner did loads of unsympathetic stuff - although they did finally add indoor facilities 😉 - such as not using lime mortar for repairs and plasterboarding over everything.
The next owners (2006-2017) only lived there for about three years before letting it out (then getting repossessed), so didn't really care for/understand what an old building needs. They added inappropriate but easily removed stuff such as glass blocks for borrowed light and polycarbonate roofing on an extension, not forgetting very mundane bathroom/kitchen fittings.
Considering it was formerly a mill (wheel etc long gone, but dried out leat and some evidence of its history remaining in the undercroft beneath the cottage), when we purchased it there was no water supply! Previously for some years this had come from a nearby farmer's well but a dispute had resulted in the supply being cut off. We installed a borehole on our own land.
Some old quarry tiled floors and a couple of Victorian fireplaces remain and we've added other reclaimed examples of the latter. We're also gradually putting some character back in, in the form of hand-painted in-frame kitchen, panelled walls, timber (DG) casement windows and period style bathroom fittings. Also using period-appropriate colours/wallpapers to decorate whilst giving the place an eclectic twist 😉 Eventually the outside will be repointed in lime. We're also creating a proper cottage style garden which it seems to have never really had.
Although three-storey and obviously a former mill if you know what you're looking at, very little evidence of its earliest history remains and it would be impossible to reinstate that aspect going forward.