England
The Planning Portal says decking does not need planning permission if it is no more than 30 cm above the ground, if it covers no more than 50% of the garden together with other extensions and outbuildings, and if no part of it is forward of the principal elevation. GOV.UK’s technical guidance defines a raised platform as one higher than 0.3 m.
Wales
GOV.WALES gives the same three conditions: no more than 30 cm above the ground, no more than 50% of the garden with other extensions and outbuildings, and nothing beyond the principal elevation. The rights apply to houses only, not to flats or maisonettes.
Scotland
Scotland uses different limits. According to mygov.scot, decking can be permitted development if it is at the back of the house, the floor is no higher than 0.5 m above the ground, and the total height including attached structures is no more than 2.5 m. In a conservation area or the grounds of a listed building, the footprint can be no more than 4 m².
Northern Ireland
We could not open the official Northern Ireland guidance when we checked, so we do not quote a figure. Ask your council’s planning office before you build.
Building Regulations are separate
Permitted development covers planning only. Guarding and stairs are covered by building regulations; see decking balustrades and decking steps.
Questions
Do I need planning permission for decking under 30 cm?
In England and Wales, decking no more than 30 cm above the ground is normally permitted development, as long as it and other extensions and outbuildings cover no more than half the garden and nothing sits forward of the principal elevation. Listed buildings, flats and some protected areas have different rules.
Is the rule the same in Scotland?
No. mygov.scot says decking at the back of a house can be permitted development if the floor is no more than 0.5 m above the ground and the total height, including anything attached, is no more than 2.5 m. In a conservation area or the grounds of a listed building, the footprint is limited to 4 m².