Q:

What does a tagline in a website header communicate by being there?

A client is asking to include a tagline in the header of their website, they are a premium agency. I lean away from this because what ever I do to try to include it, seems to take away from the premium feel of the site. 
Do I just need to try harder or is this an established design pattern in websites that infers the type of site just be being there?
I would love to see any links to sites that have done it well in a professional and high-end way.

A:

This is a very subjective answer. But I'll try.
It depends entirely on what the tagline is about.
Does it describe the work they do?
Like for example- ABC, interactive production studio.
See here
Does it say a very unique story about the brand in an original way-
Like this
How are they using their tagline in the rest of their branding material? Do you see a pattern or thought behind it that they are trying to enforce in the web design? Sometimes they might feel that their tag-line isn't coming across in a powerful way anywhere on the site. For example some agencies show their tagline as a big headline on their about page.
They are probably suggesting the header because they feel that it will be most visible there. But your job is to not find a reason to not put it there. Your job as a designer is to understand why they are asking you to this and then find a way to solve that problem rather than arguing why the header is not a good place. Maybe it could be a headline on your front-page or somewhere in the footer. Many possibilities. But they all begin by understanding your client's relationship with the tagline and how to best put it across. Hope this helped.
Some more examples- example 3, example 4

A:

I would just like to add that a good deal of companies - particularly in the software business - tend to have names that does not convey what they actually do. 
Firefox, Google, Thunderbird, StackExchange are all examples of names you have to know what they do. Seems obvious to us now, but once it was no so. 
Then a tagline/blurb might well be helpful. And necessary. "MonkeyWorks - bespoke walnut furniture". "MonkeyWorks - the news you want" "MonkeyWorks - saving the world, one pixel at a time"... and whatnot.

A:

Laura,
While tagline in the header might interfere with already established design, it will help with SEO and resulting website visibility since this text can be tagged as H1 or H2. What we did on our site www.oss-usa.com was to include small text blurbs under the header, which can still be tagged in HTML as header styles.  
I would assume that organic search engine traffic is of some interest to your client and even if they are not familiar with specific requirements of SEO optimization, they are starting to add content-driven changes to your design.  This will be a good thing for the final website, but should've been done during information architecture stage, before master layout was finalized.  You can also proactively address those kind of client changes by looking into current SEO best practices and using them as guides for organizing site and screen content on your web projects. 

