Answer
Do I need a lawful development certificate?
Not required, but useful proof your work is lawful, especially when you sell. About half the application fee.
In short
You do not have to have one to carry out permitted-development work, but it is worth considering. A lawful development certificate is the council confirming in writing that your project is lawful, which removes doubt and is valuable evidence when you sell. It costs roughly half the equivalent planning application fee, and it is most worthwhile on or near designated land.
A note
General guidance, not advice. Rules vary by site, and Article 4 directions or conditions can change what applies. Check with your local authority and the Planning Portal.
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