Q: iMac RAM upgrade - can I add ram to the 8gb that comes with it?

For RAM specifications and upgrade instructions:

https://support.apple.com/en-us/HT201191

https://eshop.macsales.com/shop/memory/iMac/Retina-5K2015/DDR3L

Yes and yes, and like Dominic23 suggests, be sure to use a quality brand like OWC or Crucial as Macs don't like some lesser quality RAM. I have done similar to what you are suggesting, I've added 8 GB of OWC RAM to the original 4 GB that was in my machine originally.

I have an iMac which needs to be disassembled to increase the Ram from 8-16Gigs.

I shudder, trying to do it myself, but I think it can be done through the Apple Store.

Anybody know 'about' how much it is ??

You may want to review the link that Dominic23 provided above. It is my understanding that the Apple stores will not add RAM to a non-user upgradeable Mac (or any Mac for that matter). An Apple Approved Service provider may though. I suggest calling one to inquire. The answer may vary from one to another. Try this link to find one near you. https://locate.apple.com/

OK, thanks.

Actually, there is an authorized shop nearby and I will consult them or some how get up the courage to do it myself. OWC has good video's on how to do it, if it's too expensive on the outside.

ttab