Guide
Do you need planning permission for external wall insulation?
For a house in England, external wall insulation is often permitted development if the finish looks similar to the existing walls. You'll usually need planning permission in a conservation area or other protected area, for flats and commercial buildings, under an Article 4 direction, or where the appearance changes noticeably. Listed buildings need listed building consent, and Scotland and Northern Ireland have their own rules.
England: houses
Alterations to a house can be permitted development under Class A of the General Permitted Development Order, but materials used on the exterior should be of a similar appearance to the existing house. Changing red brick to white render may therefore need permission, while rendering an already-rendered house, or using brick slips on a brick house, usually won't. In conservation areas, National Parks, National Landscapes and the Broads, cladding a house in render, stone, timber, plastic or tiles isn't permitted development.
Flats, maisonettes and commercial buildings
Permitted development for these external changes is written for houses, so flats, maisonettes, blocks and commercial premises normally need a planning application.
Wales, Scotland and Northern Ireland
Wales has broadly similar householder rules to England. Scotland's householder permitted development is set by the Scottish GPDO, and tenements and flats need permission. Northern Ireland's permitted development is set by its 2015 Order. In all three, check with the council before committing.
Listed buildings and Article 4 directions
Any external alteration that affects the character of a listed building needs listed building consent, and EWI is often refused. Article 4 directions can remove permitted development rights on specific streets — your council's website lists them.
Questions
Can I apply for a Lawful Development Certificate?
Yes — it gives written confirmation that the work is permitted development, which can help when selling.
Does a new colour need planning?
Repainting or re-rendering in a similar finish usually doesn't, outside conservation areas and listed buildings.
What about walls on the pavement edge?
If insulation would project over a public footway, the council's highways team may need to agree it, separately from planning.