And when there are chunks that you haven't done before?

Or when breaking it down will require going to a level of detail that means basically fully designing/writing your system in order to estimate it?

Yes! if you don't understand how something is going to be roughly designed/written then you're not estimating your guessing.

If you already know how to write it, why haven't you automated it? One could say, the worst developers are the best in estimating their tasks, because they repeat everything they do all the time. ;)

At some point you probably will want to do a rewrite. I don't think it is surprising that this sort of analytical process of bite-sized task creation also uncovers architectural deficits in your design.

That's the secret though, isn't it? The more likely you are to know all the steps the more likely you're going to make an accurate estimate. The catch there though is that if you know how you're going to do everything, you're already at least halfway through development.

It's going to take awhile to break all this down, management wants an estimate of how long these estimates will take. :\