Cascading DWT?

Suppose I have a web site with 20 areas, each of which has about 20 pages. I
want the overall design of pages in the entire web site to be the same, and
I don't want to have to change 20 DWTs when I change something in the
design, so I need one DWT for the entire web site. Yet there are also page
elements that are common to each page within a particular area. It would be
great to have "DWT children" that take all features of the "parent" DWT but
provide standard content for particular editable areas for all pages within
a particular area. Is it possible to do something like that? Other
recommendations? TIA