Planning permission relates to what and where you build. Generally if the building is:
Larger than 50% of the garden
If it is in front of your house or near a main road
Within a conservation area, AONB, has a listing or conservation order
If used for certain commercial usage
There will always be exceptions and local authority views may differ, so please do ask if you need further guidance. We have a specilaist consultant who can advise and assist with even more detail.

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.
Base heights can be in addition to this height.

Building regulations relate to how you build and can often require the same rules as building a new home. Generally do you not need them if:
Less it is than 30m2 floor area.
If it is not attached to a property.

You will need Building Regulations if:
You have water and waste connected, as this is deemed to be a habitable space.
If larger than 30m2 in floor area.
If someone lives in the building.
For certtain types of workshop and commercial areas (as fire and otehr risk factors increase).

The requirement for Building Regulations can add considerably to the design, local authority and build costs.

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.

If your building has a floor area greater than 15m2, you should build 1m from a boudary. Smaller builds can be built up to a boundary. 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.
Make sure that your building and boudary are ‘future proofed’, allowing suitable access.

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. 
Another option may be to purchase teh building personally and ‘rent’ the building to a buisiness. Tax options may allow you to charge rent as part of your home tax free.
Ask about Capital Gains Tax, as tax planning now may avoid an unexpected tax bill when you sell the property later. 
Always ask your Accountant or a specialist for current and correct advice.

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