AFAIK, it is mostly developed in-house, at least the original version was developed by the Bavarian tax office. In my experience, if government software (for schools, tax offices, political participation, etc.) is developed by an external company, the primary objective of the company often quickly transforms from "creating usable software for all citizens" to "drain the most amount of money out of the infinite resources of the federal government".

It still is developed there but they have external consultants helping them.

I have to say that the UK's HMRC web-based self-assessment site has always struck me as being a really good example of tax returns done right.

It presents as the equivalent of the paper form but with many of the fields autocalculating for you, and offering contextual help. Quite nifty.

Yeah its pretty good when you consider the complexity it has to cover.