Validity of possession certificate in bahria town

I recently purchased a possession paid plot in bahria enclave islamabad. I have got possession certificate, but there is validity of possession certificate for 6 months.
Anyone knoe what does it mean if i plan to build house later. Will they charge any extra amount after 6 months for re-issuance of letter?
I read somewhere that it is mandatory to build house within 3 yrs of issuance of possession letter. After that they impose some monthly charges. Is it so?