My family have been living in this house for at least 25 years now and we are thinking to sell it and purchase a new house,is it possible to sell a house while living in it? our plan is sell this house we are living in and purchase a new one without taking loan from bank and there is no way we can afford a new house without selling this house we are living in,the house is not in bad condition but definitely not in a condition any new buyer would move in without renovation,the house is leasehold title and house loan is paid off.

Leasehold at least need 6 months to get the title changed.
So even after signed SPA with the purchaser, u still have time to stay in before the disbursement by purchaser's Bank.

But if u need to stay longer period is better to talk to purchaser at the very 1st time u meet them and let them know how long actually u need to stay after the disbursement has been made. Consider as rent from them after the disbursement.

Easy to say is depend on both parties agreement.

can sell, why not? Ive done the same thing but once it is handed over to the new owner, you will need a place to stay.

my steps in my own case -
1. bought a new place with 10% deposit for new property which needed 3 years to complete.
2. start selling the current house which i was living in. it took quite awhile like 1.5 years or bit more.
3. after hand over to new owner I rented another place and moved out
4. waits for new place to be completed while renting

but what exactly is the question here?
are u asking whether when u sell, can u sign the SPA using the same address of the house you are selling?