Garden office build - tight to boundaries

[ScottModha]

Hi all, my first post here so thanks in advance for any replies! 

 

I have plans to build a garden office based on the below layouts with me taking advantage to get tight to the boundary. I do not live in an AONB or restricted build area. 

 

I always planned on sheet profiled steel for the sides and back as my rear fence is in a "no-mans-land" and I have rear access. The idea was that the steel would be "maintenance free". I also wanted to keep the rear access if needed for things like ladders and scaffold by means of a walk through shed, with a high security multi point locking door on the rear. 

 

Front would be clad with Cement board - Hardie or Cemberit.  I wanted to stay within PD regs, so 2.5m at the highest point of the roof, either torch on felt or EPDM and appear as my very basic front elevation drawings below.  

 

As I am cladding the outside with non combustible materials, and I am under 30m2 am I correct in assuming

 

1. I need no building regs 

2. I need no planning permission and can proceed under PD regulations 

 

Do I need to consider anything else?

 

I am also having second thoughts on digging down by 150mm to allow me to get some extra internal height as I wanted to use this room not only as an office but for my indoor bike trainer.