<p>Background:
I am developing a site in Asp.net 2.0 . Up until now i was handling both the design and development of the site. I used css for the design part.
Now the company wants to outsource the design work to a web designer.   </p>

<p>Question:
How exactly are a designer and developer supposed to coordinate 
What specifications should i give to the web designer, Do i need to provide him with the aspx pages i have developed, does he also need to look at the code behind. </p>

<p>And how do i incorporate the designed pages in the existing aspx pages.</p>

<p>I would appreciate if someone who is experienced in this provides some insight</p>

<p>thanks.</p>

How will the designer deliver their designs? Will they just provide the necessary graphic elements or will they deliver a valid HTML page? I would suggest you both agree on the DOM for the page and work from there.If they just provide graphics you have full control of the DOM.
The code behind model for .Net was supposed to help separate design and development but IMHO nothing has yet managed a perfect separation because you can't always make a 100% distinction.