Demolish utilty and rebuild: planning permission ?

[Nice But Dim]

There is a 1.8 x 4.3 m single story flat roofed utility room stuck on the end of a larger kitchen extension also flat roofed which were both built pre 1999

The utility is only single bricked and starting to slip into small brook which runs down length of outer wall

Can I just demolish the utility and rebuild to current building regs specs ? Or would it need planning permission ?

And if it does and I dont apply for it what is the recourse ?

Thanks for your counsel

[Nice But Dim]

Thank you all for your comments

Both the kitchen and utiliy extension were done under formal palnning permission not a permissabke development pre- 1999

The house has already been expended to its max cf. its original curtilage

I woukd have thought that if the utility was rebuilt to the same plan and footprint then there should in theory be no problem in gaining formal permission for the work. What am I missing ?

(I am not too concernred over the potential of significant deeper foundations due to the brook - which runs dry over the summer months - although 2 m sounds excessive !!)