Hello

I am looking for some advice on building a garden building!
I have read the threads about garden offices but this is on a larger
scale!!!

We have a large garden that is split on two layers with the top garden
(the smallest) extending from the house.

we are hoping for a brick/block built building measuring about 7m x
12m.
For this the top garden will be dug down to the level of the bottom
garden and the foundations will be placed! The building will be on two
levels with the bottom being a garden shed / bike shed /workshop and
the upper layer being a gamesroom/ bar/ gym.

what would be the best way to get this project going?
I have never been though this process. do i commision an architec? or
just get some outline plans drawn (by who) and submit these to the
local planning office?

cheers

anthony