Obviously, you will need Epiphany installed, and all of its required libraries. Development headers must be installed, too (for most distributions, you must install both the regular library and the -dev or -devel packages— but if you don't know this already, just give up now and save yourself the confusion). You will typically need GTK, GLib and Epiphany development headers as a minimum; depending on what your extension does, this list may be much larger.

This document suggests features only found in Epiphany SVN; if a particular feature does not work for you, it could be that your version of Epiphany is too old.

It is highly recommended that you install epiphany-extensions as well. Besides providing a user interface to enable and disable extensions (so you don't have to edit the GConf keys manually), it also provides a C build framework.

To compile your extension with the existing C build framework, you will need GNU autoconf, GNU automake, libtool, gettext, etc. Basically, you must be able to compile Epiphany from SVN. In fact, you might as well install Epiphany from SVN before coding.

If you want to write your extension using Python, you should still install epiphany-extensions for its extension enabling user interface. You must also install pyphany, which includes Python bindings, a Python script loader, and some sample extensions.