Replace garage bigger

I have a external stand alone garage which is 6m x 2.5m, 2.2m high with a flat roof and would like to replace it with a 8m x 3.5m with a 3.5m gable roof, one of the existing walls it the on the boundary to next door as would the new one be, do external buildings come under the new permitted builds or would I have to have planning permission? It is on the rear of the house. Next door do not have a problem as they are extending their house along the same boundary I would just like to know where I stand legally please and or what to do next?

 

It’s classed as an outbuilding of which the PD rules are here...

https://www.planningportal.co.uk/info/200130/common_projects/43/outbuildings

 

First check if your house actually has permitted development rights.

And be aware of the building regulation criteria if you want to be exempt from building regulations (which is different to planning regulations)

And building over drains have rules

And Party Wall Act may apply