Sadly you can't just build what you like.
Decking taller than 30 cms high requires permission!! This causes issues on resale and you may need to buy an indemnity policy to get around this.
Your installer will not apply for planning. It is the landowner's responsibility to do this.
Enforcement action means you could be ordered to tear it down and pay costs
Read more here.
The Rules for BOTH planning & building regulations for common alterations are found here ...