This is why it's easier to build a nuclear bunker in your garden than a contemporary home extension... ...at least from a planning perspective... Permitted Development (PD) in the UK is a legal right allowing many property owners to carry out certain works without having to apply for planning permission. It's great in many ways. People have the freedom to build without having to ask for permission from their local Planning Department, an application that can meet resistance and or refusal. A keen DIYer, though, can legally cover 50% of their entire garden with new outbuildings and can even build a nuclear bunker beneath it, all without any application or oversight whatsoever, using their PD rights. Under 30 sq m you do not even need building regulations sign-off... Yes, you really can build a structurally questionable nuclear bunker on your own, with no experience and no oversight (OK, you may need a party wall surveyor if you make it deep enough to actually be... useful). Just remember, you cannot make your outbuilding an independent home or attach it to yours. Maybe a tunnel is OK, but that is a grey area; it would be difficult for the council to prove anyway... Without the tunnel, you are good to go; get digging... Your outbuilding can be clad in pretty much anything you like. Let's say, for the sake of argument, you like timber and glass. Done... So, you have built your concrete bunker and your enormous outbuilding and clad it all in timber (and glass), but then decide you would like to extend your house with a small glass and timber-clad extension. Do not be silly goose; you will clearly need planning permission for that. What were you even thinking?! To the naughty step with you... Let's just hope your planning officer likes timber this week... Does anyone else think this is daft? Or is it just me?
(Don't worry; you can usually extend your house under PD, too, but if you add some design flair, planning is likely still required and is usually achievable with the right guidance... Perhaps it's worth considering that timber and glass-clad nuclear bunker after all, though, as it could be built by the time your extension gets its permission... 🙃)