Balcony on flat roof

Samophrah

Hello,

I am currently in the middle of planning a refurbishment of my flat, and I'd like to check on the steps that I need to follow to build a balcony on a flat roof following all legal regulations. The balcony would be built on top of the flat roof that his extension downstairs has. 

Context: the property is split into 2 flats, of which I own the top floor one. The freehold is split between the landlord of the ground floor flat and myself. 

We pay as you go on expenses for the property (building insurance, basic maintenance, etc).

We are both directors of the freehold company. As far as I am aware, there are no rules or articles applying on the freehold registered anywhere. 

Other than requesting planning permission and hiring an engineer (hoping that I do get the permission to build), what legal aspects with regards to the property ownership and agreements with the landlord downstairs should I take into account? What are the steps to follow, and do I need a lawyer?

The landlord downstairs doesn't live in the country and, while I can contact him, I am not sure if he has a legal representative. By the looks of the post he receives, he uses the address as his main address instead of having it diverted. 

Thanks!