Playroom (I know wgas)

[MarillionFan]

What, cables everywhere, tissues on the floor, a pile of "for dummies" books in the corner and a few VAT late payment demands scattered around for good measure?

NorthernLad is the new James Khan.

[PAH]

Do I need planning permission?

For the changes you're making to the structure you just need to worry about the normal rules on extensions or detached buildings on your own land. i.e. similar rules to those if building a garage or large shed. Main ones are to do with height restrictions and how close to boundaries or main building they can be.

Otherwise you may need permission if changing use to an advertised commercial premises (signs outside or obviously commercial) where you expect visitors or will be storing certain dodgy things such as flammable liquids or toxic chemicals. Will then be liable for commercial rates and other nonsense.

If it's just a work from home office then dual use for tax/expenses is likely, but some people don't bother with that and just keep things quiet.

Depends how public it will be and how much of a stickler for the rules you are.