Putting up decking, or other raised platforms, in your garden is permitted development, not needing an application for planning permission, providing:
In some areas additional limitations may apply. If you are unsure contact your local planning authority.
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The permitted development allowances described here apply to houses and not to flats and maisonettes.
Building regulations should be assumed to apply to every deck structure requiring planning permission.
If you are unsure about whether you are required to comply, you may wish to contact your local building control body.