In reply to matt30987:
Hi Matt
Forgive me if this is teaching you to suck eggs, but there are two main things to think about with outbuildings: planning regulations and building control.
If you're wanting to build an outbuilding in your curtilage (i.e. your garden/grounds) and don't want to have to apply for formal planning permission from the Council, then you'll want to construct using the deemed permission of the Permitted Development allowances (aka your PD Rights). These dictate all the rules about how much development you can lawfully do and will give you all the measurements about area, heights, proximity to neighbours, etc. This document would become your bible: https://www.gov.uk/government/publications/permitted-development-rights-for...
If you want to know whether what you plan is lawful before you go to the time and expense of building it then you can to either an informal planning enquiry to the Council, or a formal application for a Lawful Development Certificate (LDC). The latter is more expensive and takes longer, but gives legal protection. The rules are subtly different between England and Wales.
Building control is different and entirely separate within council administration from planning. It's about making sure that buildings are well built and safe. The costs of getting someone to supervise building work to make sure it satisfies building regulations is not that great. Definitely cheaper than formal planning permissions, building plans, etc.
I wouldn't put my effort into avoiding building control, but I would make sure I'm squeaky clean on the PD rights. And obviously, if you want to get on with any neighbours afterwards, talk to them about what you intend to do beforehand! Neighbour disputes can become a total life misery.
Sounds like a fab wall. Enjoy, Tony