Are there Human Interface Design Guidelines for Unity Development?

From Ubuntu Apps Developer Documentation, I guess these are the guidelines you are looking for:

• Ubuntu Platform overview and Unity Desktop Platform

• Application indicators design guidelines

• NotifyOSD design guidelines

• Application menu presentation

• Ayatana scrollbars presentation

• Sound menu spec

Also refer:

• Are there any standard UI / UX / design principles or guidelines for building Ubuntu apps?

Design guidelines for Ubuntu Touch apps (e.g. mobile & tablet) are provided by Canonical:

http://design.ubuntu.com/apps

But still nothing for the desktop. Many suggest that GNOME's HIG are the guidelines for Ubuntu's desktop interface, but those guidelines are suited for the GNOME-3 desktop which is pretty much different from Unity.

Canonical is recently too much busy with Ubuntu Touch & Ubuntu Edge projects, which has turned it's focus to mobile platforms. But maybe in the future, we will see some attention returning to the desktop sector ... hopefully!