Better html structure for putting the header and sidebar in left side column

[admin]

In this <a href="http://www.codetocode-developments.com/personal/" rel="nofollow noreferrer">site</a> I have a wordpress theme with this HTML structure:

<header>
    <nav> ... </nav>
</header>

<div class="post-content"> ... </div>

header {
float:left;
with:30%;
}

.post-content {
float:right;
with:30%;
}

And I want to add widgets to the left column.
My options are:
1)make header widgetable: widget will appears before content and this is bad for seo.
2)put the header and the sidebar in a wrapper: the same, widgets before content
My question is: is an html bad practices to put header inside other div? And put the header after content in the html? What is the best structure for doing it?