These were the rules but it's worth checking with the council in case they have changed recently before relying on them. They will be happy to tell you.
You cannot build on more than 50% of the area around your house (this includes outbuildings and any extensions) without requiring planning permission. Any shed or outbuilding must be for "a purpose incidental to the enjoyment of the dwellinghouse" which means that for example you cannot rent it out, use it for business purposes (including storage) or use it as a separate house without applying for planning permission.
You will need to apply for planning permission if the outbuilding is higher than:
4m if it has a dual-pitched roof,
2.5m if it is within 2m of the boundary, or
3m in any other case.
And wherever it is built, the height of the eaves of the building cannot exceed 2.5m (if it has eaves, flat roofs do not).
If your house is in a conservation area you will need to apply for planning permission to build an outbuilding to the side of your property (even if it is in your back garden and not directly to the side).
If your property is a flat or maisonette (including those converted from houses) or a commercial property, such as a shop or public house you will need to apply for planning permission.
If you live in a listed building, you will need planning permission for any outbuilding and may need listed building consent for any significant works in the grounds (curtilage).
Edited to add: Probably more of a hurdle (as the rules for once are quite generous it seems) are the issues of potential nuisance claims from any near neighbours re: smell, muck heaps etc.