Hi not sure if anyone can help we are having a small extension (for a plant room) and making some alterations to the property change of Dormas, entrance location change. 
 

obviously I understand that part L needs to be complied with on that part and we are actually also thermally improving the roof as part of the works. 
 

but am now being advised that I will most probably have to externally insulate the rest of the property or carry out internal insulation measures throughout the property. 
 

is this correct? 

Doesn't sound correct to me. You only need to upgrade if more than 50% of the existing structure is being modified and there is a get out clause which says "where technically, functionally and economically feasible."

 

External wall insulation is probably a good idea if it's practicable but I doubt that it is enforceable under building regulations.
 

Who has advised you that… I assume not Building Control as it is false information. As mentioned above, you would only be asked to carry out further upgrades should you be removing, replacing or renovating more than a certain percentage of the element or fabric…

 

https://www.planningportal.co.uk/permission/common-projects/roof/building-regulations-insulation-and-thermal-elements