Your mother can only sell her share in the property if the property is held under tenancy-in-common. If it is held under joint tenancy, then it is more complicated.
A joint tenancy can be severed (cut or split) and converted to a tenancy-in- common by an instrument of declaration.
A joint tenancy can be severed by the following:
1. By alienation: which means by selling away one’s share making the joint tenancy a sole ownership or tenancy-in-common. For example, one of the co-owners need money, sells his share to his other joint tenant, such that it’s now owned by one owner, sole-ownership.
2. By partitioning by mutual agreement: means breaking the joint tenancy by way of mutual agreement.
3. By partitioning via court order: e.g in divorce where the court will instruct the divorced couple to dispose of the marital home and split the sale proceeds according to a certain apportionment by percentage. This effectively breaks up the joint tenancy into ownership by percentage shares.
Your parents can go to HDB's office and pay an administrative fee to request for this transfer, provided all parties are agreeable to this. This is the more affordable option as compared to getting a conveyancing lawyer to do it for you. In the form, you will have the option to base your flat valuation based on the recent market transaction for similar HDB flats in your area. You may refer to HDB's link here: https://services2.hdb.gov.sg/webapp/BB33RTIS/BB33PReslTrans.jsp?fbclid=IwAR3gGOuYjgjyhCm6kgkHcjSh2OdKxELcs-kmTssf91OyWwIfqEE_jzkpehg
It can also be based on the median price in your HDB estate. You may refer to HDB's link here: https://www.hdb.gov.sg/cs/infoweb/residential/buying-a-flat/resale/getting-started/resale-statistics
Your mother may transfer her share to you provided you meet HDB's eligibility conditions. You may refer to HDB's link here: https://www.hdb.gov.sg/cs/infoweb/residential/living-in-an-hdb-flat/changing-owners-occupiers/change-in-ownership/transfer-of-flat-ownership/eligibility
You must also return the CPF amount used plus accrued interest that your mother have used towards the current HDB flat. On top of that, you must be able to able to service the mortgage.
On your mother's end, she must settle any outstanding amounts to HDB such as fire insurance premium, late payment charges or bill charges before the application for transfer can be considered.
Hope this answers your question.
You may purchase your mom share using the resale of part share scheme. You may want to arrange an appointment with your HDB branch as approval for such schemes is subject to HDB. You may also contact me at 93375051 for any further clarifications and I am more than willing to assist.
ERA Realty Network Pte Ltd