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The usual path for conjugation would be:

  1. 高い → 高すぎる (drop い, append すぎる)  

  2. ない → なさすぎる (drop い, append さ, append すぎる)

Is ない in つまらない treated as the negative ない which conjugates as 2.?  
Or is つまらない a single piece that conjugates as 1. ?

Does つまらない conjugate to つまらなすぎる or つまらなさすぎる? Are both acceptable? If so, is
there a preference for one form or the other?

Do the conclusions that apply for つまらない also apply to similar adjectives
ending in ない such as:

  * 危ない (ない explicit as おくりがな)  

  * 汚い (ない hidden inside the word)
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grammar conjugations adjectives negation
ない in つまらない and similar adjectives and their conjugation with すぎる
~~The rule is that さ intermediates when the word stem consists of only one
mora (e.g. な as in ない and よ as in よい).~~

~~The stem of つまらない is つまらな, which consists of 4 moras, therefore~~ the
orthodox one is つまらなすぎる.

That said, つまらなさすぎる is also accepted in practice. ( **edit** : Some people
probably find it wrong.) As for preference, I find both of them almost as
frequent as each other.

As for 危ない and 汚い, あぶなさすぎる and きたなさすぎる are not used.
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Dictionaries say すぎる in this meaning is placed:

  * after the 連用形 of a verb, like 動きすぎる
  * after the stem of an i-adjective, like やさしすぎる
  * and after the stem of a na-adjective, like しずかすぎる. 

つまらない is an adjective so I think つまらなすぎる is natural.

Generally, when すぎる is placed after ない:

  * in the case of the adjective ない, it uses さ, like なさすぎる as you say
  * in the case of the negative form of an i and na-adjective, I think it uses さ, like つまらなくなさすぎる, しずかでなさすぎる.
  * in the case of the negative form of a verb, すぎる is placed after な, like 動かなすぎる. 

However, in the case where the stem of a verb is one character/mora, さ is
often placed after な, like しなさすぎる, 見なさすぎる.

This さ is said to be an additional strength for the instability of one-
character (single-mora) stems.

It is written here in detail:
[「なすぎる」？「なさすぎる」？](http://d.hatena.ne.jp/higonosuke/20130710)

In addition, ない is basically following.

・in the case of the adjective ない which means "not be" like 卵が無い.

・in the case of the adjective auxiliary ない which means "negative" and is
mostly placed after adjective like 美しくない.

・in the case of a part of an adjective like あぶない.

・in the case of the verbal auxiliary ない which means "negative" like 動かない

However there are some persons who said the adjective auxiliary ない is same as
the verbal auxiliary ない because both means "negative". In detail here.
<http://okwave.jp/qa/q2762974.html>
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As both Yuuichi Tam and user4092 have noted in their earlier posts, the
pattern for すぎる after -ない can vary. Part of this is because the -ない ending
itself has two derivations.

  * One is from the negative 無い. (Historically, it's more complicated, but in modern Japanese, the negative ない suffix is functionally the same as standalone 無い.) The adjective つまらない _"boring"_ evolved from verb 詰{つ}まる, originally in reference to a story or circumstance that _wouldn't_ つまる or finish up -- imagine someone long-winded who just won't get to the point. Adjectives like this that are based on negative 無い can often take the -さ- between the な and the すぎる.
  * The other ない ending appears to derive from, or at least be cognate with, old auxiliary verb なう that expressed repetition or ongoing state, like in verbs 行{おこな}う _"to carry out, to perform"_ or 商{あきな}う _"to do business, to buy and sell"_. The ない in adjective 危{あぶ}ない is this other ない, not the one meaning _"not"_. Adjectives like this generally _don't_ take the -さ- between the な and the すぎる.

So modern つまらない can apparently form either つまらなすぎる or つまらな **さ** すぎる, while
危ない and 汚い should only form 危なすぎる and 汚すぎる.

**Note:** there is some debate about this among native speakers. Textbooks
make a distinction between ない on its own to describe the lack of something
(like 卵{たまご}が無{な}い _"there are no eggs"_ ), which does take the -さ-, versus ない
as a verbal suffix to indicate the negative, which shouldn't take the -さ-. But
it seems that the people on the street might not make this same distinction,
or perhaps they are making it less as time goes. Google searches do turn up
examples of
[危なさすぎる](https://www.google.com/search?q=%22%E5%8D%B1%E3%81%AA%E3%81%95%E3%81%99%E3%81%8E%E3%82%8B%22)
and
[汚さすぎる](https://www.google.com/search?q=%22%E6%B1%9A%E3%81%95%E3%81%99%E3%81%8E%E3%82%8B%22),
for instance, albeit with fewer hits than the expected forms without the
interstitial -さ-.

See [this Chiebukuro
post](http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1014406135) for
some discussion of the phenomenon.
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