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                                                                     Exhibit 5.1

                       [LETTERHEAD OF HADDAN & ZEPFEL LLP]

                                  June 18, 1999

Modtech Holdings, Inc.
2830 Barrett Avenue
Perris, CA 92571


Dear Sirs:

         You have  requested  our  opinion  with  respect to certain  matters in
connection  with the filing by  Modtech  Holdings,  Inc.  (the  "Company")  of a
Registration  Statement  on Form S-8  (the  "Registration  Statement")  with the
Securities and Exchange Commission, covering the registration of up to 1,181,163
shares of the  Company's  Common  Stock,  $0.01 par value  (the  "Shares"),  for
issuance pursuant to the Agreement and Plan of Reorganization and Merger,  dated
September 28, 1998.

         In connection  with this opinion,  we have examined and relied upon the
Registration  Statement,  the Company's Certificate of Incorporation and Bylaws,
and the  originals or copies  certified  to our  satisfaction  of such  records,
documents, certificates,  memoranda and other instruments as in our judgment are
necessary or appropriate to enable us to render the opinion  expressed below. We
have assumed the genuineness and  authenticity of all documents  submitted to us
as originals,  the  conformity to originals of all documents  submitted to us as
copies  thereof and the due execution  and delivery of all  documents  where due
execution and delivery are a prerequisite to the effectiveness thereof.

         On the basis of the foregoing, and in reliance thereon, we are of the
opinion that the Shares, when sold and issued in accordance with the
Registration Statement will be validly issued, fully paid, and nonassessable
shares of Common Stock of the Company.

         We consent to the filing of this opinion as an exhibit to the
Registration Statement.


                                        Very truly yours,

                                        /s/ Haddan & Zepfel LLP
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                                        Haddan & Zepfel LLP