It all depends on what type of Extension you are planning and the additional space size you have at the back of your house.
Extension for both floors of your house will cost you more as you need to have a solid foundation and beams erected and you will have aconcrete base. This could easily cost you RM40k and above depending on the size and if you are moving your toilet from its original loaction.

Just for the extension of your kitchen will not cost a lot but again depending on your size. But it would start from RM15k.
The most important thing is getting a good contractor . You have to ask them what are the iron size that they will be using for the foundation and how deep will it be. You need to talk to them as if you know what is needed to be done.
BTW- Never ever pass payment in full to the contractors.
Always go for 30% after initial job is started.
Another 30% after 3/4 job is done.
20% when job is completed.
Last 20% few weeks after job completed . This is so u will have time to check for any leaking or cracks.

Hope this helps