My grade two listed property had an extension to the kitchen built in 1987. I assume that this is also listed as it is part of the early victorian body?
Although that isn't what the DCMS seemed to feel when it wrote about curtilage on the website last year!
And it still isn't simple - I'm actually in the middle of an objection to ruining a Conservation Area building of note (all in the wider public interest of course) by altering it beyond recognition (no Article 4 and of course post-Shimazu...) and I think there is a strong case to be made that it could be deemed listed Grade I as curtilage of a structure to which it is an ancillary building - a Church Hall. It's complex but it's all on the same piece of ground for statrers. A fellow local stirrer has written to EH but has as yet had no reply. (Physical layout, ownership past and present, use and function past and present - all of course subject to 'a matter of fact and degree!).
Attorney General ex.rel Sutcliffe v Calderdale; and Court of Appeal Debenhams v Westminster Council - a structure must be subordinate to and and ancillary - also comes into play.
Of course although the theory that all quality additions over time are listed holds, some additions may be deemed less valuable than others and so may be able to be altered.
There are cases where permission has been given (controversial) to remove Victorian wings from earlier houses for example. It really is to be decided building by building, but sense and right doesn't always prevail.
Then we have Span 4 at Paddington - a sensitive and carefully engineered addition to the Brunel spans - derided by 'consultants' for NR as pastiche - currently under threat of demolition despite its Grade I listing - all of course in the wider interests if the public, and nothing to do with Network Rail's desire to make money from office blocks over new tracks.
If this goes ahead it will be the biggest Grade I demolition since listing began.