Double extension

[ArseEndofNowhere]

We’ve looked at a house which could be perfect if we can put a big double extension across the back. I know the rules say you can build 3 metres from the back wall of the house without planning permission. We want to knock down the study and build the extension about a metre further out. The study looks to be an old extension with a fireplace, probably added in the 1940s or earlier- would this count as the original wall or not? I know anything built or added before 1948 is counted as original for planning.
We’ve not done an extension before as you can probably tell and totally confused about where to start 😂

[Rollercoaster1920]

If it is unclear you'll need to apply for full planning I expect. I wouldn't take the risk not to.