Hi -- The regulations continually change and what falls into permitted development without needing PP is shortly due another change, a lot depends on the size of the proposed extension in relation to the existing floor area of your home or if there have been other additions that have not needed PP.
I would advise speaking to the neighbour and phoning your local planning dept, they are usually very helpful and will give you an idea of all the process and whether you need PP in your particular case, building regs is vital and detailed plans have to be submitted to the building control dept and passed before any work is carried out.
I enclose the national planning portal website that gives you general help.
http://www.planningportal.gov.uk/england/genpub/en/1011888239467.html