what is turnkey website ?
The reason for a turnkey site is a plug and play kind of site that’s almost ready for the new owner to host and start his web presence or sell product that comes with it.
Not sure why you are asking about layout change - you mean you trying to tweak an existing site so that the end product can be called your own?
Layout changes depends on the scripts and familiar with MySQL, etc., and your experience in web development in general.
What’s your purpose or intent for a turnkey site?
its usually a script ready to be used on a website, you dont need to change anything or add content.
The only thing you need to do is upload it, point a domain name to it and start your marketing.
It can save you a lot of time develloping websites, but your site won’t be original.
In one word, Turnkey site means those sites which can turn into a key feature for a source of income 
Regards
hmmm … thnx for answers …
but can they be changed , i mean is changing ther elayout etc is too difficult ?
It depends on the developers specification. If the developer has used a easy system for the turnkey sites can you should get enough flexibility while redeveloping it. But most of the time they are enough flexible to use. The sellers give enough specifications and time limit, upto then he helps you out on the turnkey sites.
Checkout the Turnkey Sites selling section of SP. the sellers give some specifications which might help you out.