Do i need to declare room rental as part of my income tax?

Yes room rental is part of your income.
2. U cant deduct... Anyway when u rent out, the amount is submitted to HDB.. They have your records

Yes you have to.

"This includes rent of the premises, maintenance, furniture and fittings. Rental income is subject to income tax. This means that any profit or net amount left once you have added together your rental income and deducted any allowable expenses is taxable."

Do check out IRAS' website for more info.

Source:https://www.iras.gov.sg/irashome/Individuals/Lo...