Can I build?

You've kind of summarised it with your "ask your local authority" point at the end - whilst the law and regulations are essentially national, interpretation (and in particular, the extent to which a given câmara will make an issue) varies widely.  This, coupled with the fact that precedence is not a meaningful legal basis here, means others' experience is broadly irrelevant to anyone else's situation - what matters is the letter of the law, and then the way that is enacted in relation to your specific property.

In very broad generalisations, temporary structures which are not firmly/permanently fixed to the ground are not likely to cause too much hassle.  Your point 1 may be classed as that, but if it is heavy/anchored/clearly intended to be permanent, this will not be the case.  Your point 3 sounds intended to be a building, not a temporary thing, so is likely to need permission (and don't assume you'll get it, discuss first with an architect, as if planning is required, a project will be required).  Point 2 is probably more about compliance with regulations, than planning, though the wording is unclear to me.  But as mentioned, this is an informed "gloss", as no-one can give you definitive advice about your property without much more detailed knowledge of the location, land, buildings, etc.

there used to be an advert for the AA in the uk...its copy line was "the answer is yes - now whats the question"

perhaps in answer to your posting you should keep in mind the amended copy line "the answer is no - now whats the question"

Hi James,
And here is an Architects two cents on this:
1. Any structure with connected services needs a planning application, whether on rustic or urban land.
2. Yes, generally, local authorities in Portugal are not concerned with internal layouts so long as you don’t change the external facade in any form or shape. Keep in mind that if you don’t go through planning for new internal spaces, they cannot be legally included in the information of the house, as they’re not officially legalised. Always get a structural engineer to double-check where and how the builder wants to open connections between levels.
3. Yes on urban land with a planning application, and no on rustic land. A functioning workshop, office or gym will need some services like electricity. However, some councils allow you to build an additional 25m2 on rustic land with a planning application. It depends on the council PDM and other regulations.
All said, planning applications and licencing are not very complex. The architectural cost of getting a planning application and having everything you’ve mentioned legal (a hut kitchen/ bbq space, internal stairs, a space for workshop/ gym/ office) on urban land is between £1400 - £1900. Given you will need at least twenty thousand to do the above, getting everything done legally adds to the property’s value.
Hope this is helpful, and let me know if you have any other questions,

Vera

I suspect my next step will be to find a highly recommended architect in the Ferreira do zezere area that I can talk to. Any recommendation greatly appreciated.