Hi Mdm Iris Ng, thanks for writing to us on AskGuru.
Do note that for HDB flats and private properties, there is a minimum rental period allowed. For HDB flats, it is 6 months. You may refer to HDB's link here: https://www.hdb.gov.sg/cs/infoweb/residential/renting-a-flat/renting-from-the-open-market/regulations-for-renting-out-your-flat
For private properties, it is 3 months. You may refer to URA's link here: https://www.ura.gov.sg/Corporate/Media-Room/Media-Releases/pr17-44
Anything less than that is a serious infringement of HDB's and URA's regulations.
In your case, it is advisable to renew the lease two months before the expiry of your tenancy agreement.
There are no costs involved for such a lease extension except for a $20 administrative fee for subletting approval from HDB.
Please log in to HDB's website using your SingPass to do so.
Hope this answers your question.