Yes, once any part of the deck's floor height exceeds 30cm (300mm) above the ground beside it, planning permission is required before you build. Below that height, and within the wider 50% garden-coverage limit behind your home's main wall, no application is needed.
That 30cm line matters more here than in a flat inland town. Alum, Middle, Durley and Boscombe chine gardens, and the gardens stepping down toward West Cliff and East Cliff, rarely sit level, so a deck that looks low from the patio door can still clear 300mm once it's measured against the highest point of the slope beneath it. Get the measurement checked before you commit to a design; my decking planning permission page covers the BCP rules in full.
Where is the 30 cm measured on my sloping or chine-side garden?
From the highest point of the ground next to the deck, not the lowest. The government’s own permitted development technical guidance states that where ground level is not uniform, “the ground level is the highest part of the surface of the ground next to the building”, and that ground level “would not include any addition laid on top of the ground such as decking”. On a sloped chine garden that means the uphill edge decides the height, not the downhill one. A deck that sits flush at its lower end can still be well over 30cm at the top if the ground falls away beneath it, which is the detail that catches people out on Bournemouth’s steeper gardens.
Does my lease or freeholder consent still matter if planning permission is not needed?
Planning permission and your lease are two separate questions, and clearing one does not clear the other. If your home is leasehold, or a freeholder's covenants restrict what can be built in the garden, that consent sits outside Permitted Development entirely and needs checking against your own deeds. I could not find a published planning rule that settles lease or freeholder consent either way, so treat it as a check on your own paperwork rather than something the planning rules decide. Check your lease, and your freeholder where one exists, before work starts regardless of what the planning rules allow.
If you want a straight answer for your own garden, ring Alfie on 07841 319560.
Related: back to planning permission for decking and pergolas: the questions Bournemouth homeowners ask.
Related: do decking permitted-development rights apply to my converted flat or maisonette?