Let’s consider what is useful for your visitors (especially those that rely on the headings/outline, e.g., screen reader users), and forget about "SEO rules".
Whether you want to follow HTML5’s outline algorithm (see tools), or if you just rely on the heading elements (without taking sectioning content element into account), it makes sense to think of it like a ToC.
Your homepage has this outline currently:
1. Document
1.1 Navigation
2. "Seit über 35 Jahren"
3. "Naturstein – unsere Leidenschaft."
3.1 "Das Unternehmen"
3.2 "So arbeiten wir"
3.3 "Referenzen"
3.4 "Weitere Objekte"
Is this helpful? I’d say the "Seit über 35 Jahren" section shouldn’t be there. If a user navigates to this heading, there is nothing there -- no content, no sub-headings. It should be part of the content; it should not be a heading. Adding it to the following heading might be possible, but it seems to be more appropriate to make it a subtitle/tagline, e.g.:
<header>
<p>Seit über 35 Jahren</p>
<h1>Naturstein – unsere Leidenschaft.</h1>
</header>
That said, is "Naturstein – unsere Leidenschaft." really a useful heading? It seems to be a slogan, or an introduction, but wouldn’t it be more useful to have the actual name ("Natursteinwerk Villmar") as heading? If I’d mark up this site, I’d go for one of these outlines for the homepage:
If using the site name (which could also be the logo with alt
) as document heading (for every page; see also):
1. "Natursteinwerk Villmar"
1.1 Navigation
1.2 "Das Unternehmen"
1.3 "So arbeiten wir"
1.4 "Referenzen"
1.5 "Weitere Objekte"
(Where the last four sections could be part of the main
element. The visual design could stay the same, so "Naturstein – unsere Leidenschaft." could still be in that big font size; the only difference being that it’s now a p
.)
If not using a document heading:
1. Document
1.1 Navigation
2. "Natursteinwerk Villmar"
2.1 "Das Unternehmen"
2.2 "So arbeiten wir"
2.3 "Referenzen"
2.4 "Weitere Objekte"
If you know which outline you want to have, choosing which heading elements (h1
-h6
) to use is easy: the first level gets h1
, the second level gets h2
, … -- headings represent rank, not importance.