Up to 2.5 meters high without planning permission needed. Be mindful of garden heights though.
If further than 4 meters from a boundary, you can build higher.

Up to 30 square meters of floor area usually does not need permission. Up to half the area of a garden.

We comply with Electrical regulations, but no need to follow others usually unless you have water and waste plumbing, or plan to use it as a place to sleep and live.

As a garden building there is no rule, but we still use safety glass everywhere and A+ rated double glazing for insulation with background ventilation.

Whilst we may be able to build close to a boundary, our team will need space to work when they are creating your garden building. Then you need space to clean and remove any debris. We normally suggest 60cm. Any closer, you may need to remove and refit fence panels with neighbours consent.

Your local Tree Protection Order Local Authority Officer can give specific advice. But generally, our ground screws base option is the most preferred. They can avoid the damage to roots caused by strip or slab concrete bases.

If the building is solely or partially for business use, your Accountant can advise if and how much you can offset as a cost against earnings. Do not forget about any specific pathway, interior finishes and furniture too. Ask about Capital Gains Tax, as planning now may avoid an unexpected tax bill when you sell the property later. 

Easy, visit this Government website for all current rules. Also, give a very detailed and simple to understand explanation here.