Hi

a number of leaseholders are receiving letters from the management company informing their front doors (10yrs old) are no longer compliant with recent building regulations for flats and it is the leaseholders responsibility to urgently replace. Apparently a gap of 3mm is the limit and most are 1mm over. 
 

My query is not about who is responsible but what are the options? It just seems so wasteful 100s of perfectly newish doors will go to waste and admit not quite understanding the regs is there a retro option to meet the alleged regs? 

Firstly it's up to 4mm usually and there has been no change in building regulations affecting flat front doors and in any case the Building Regulations do not retrospectively apply to existing buildings unless being altered.

Official guidance for front doors in flats allows varying standards of door dependant on the size and layout of the block - there is a fair chance no doors need replacing.