I read almost all post about #agile tag on dev.to before asking this.

Recently while working on a dummy project (yes, a dummy project just to understand the flow), I read a good post from swarmonline discussing project estimates between waterfall and agile. (PS: I am not affiliated with them at all).

Someone wants to quickly start a project that might have 20 features and take 200 hours, client is also not sure about the total budget, but it might be say $5000.

The discussion follows with several written document/mail/video call etc to understand the project requirements.

After that, someone has to write a detailed requirements so the client does not ask for excessive changes outside project scope. However one cannot be too detailed and specific ending like a waterfall project which has most specifications set from start.

A good specification for that 200 hours project can take 8 hours, while a rough estimation might take 2 hours, or just 15 minutes with even smaller one.

How much detailed the specification should be for an agile project?