What would add more ROI? Additional Room options

I am currently tweaking my plans and there is an opportunity to utilise my sloped block to add an additional room under the house.

The easiest and cheapest option will be by accessing it externally from the very rear of the house. My concern here is its value add as technically I am unsure what it would be classified as? Calling it a bedroom may be a stretch as you would need to go outside to access the main living and bathroom? I am
Thinking man cave.

The second option is changing around some of the internal layout and accessing it from inside. This is a more costly exercise but then brings the room inside where it can definitely be a bedroom/study.

I have gone back to my builder for a ball park. I have a feeling the difference between the two will be about 20k.

Thoughts?

 

BIL had a house with a reasonably large single room underneath on a sloped block. He had a desk and some bookcases/filing cabinets in it, so it was a genuine study for a guy working from home, but it also comfortably fitted a bed, so also worked as a spare bedroom whenever guests came over. Worked for a boarder, but not really very well if it was a young family member living in it. "Detached study" might work for you?

 

If you are going to go to the expense of another room, you may as well add a small bathroom and kitchenette.

Would certainly improve potential yield, particularly if access from street is good.

May not be as costly as you think, particularly if services such as sewer pipes and water pipes run nearby anyway.

Get the pricing done with and without bathroom/kitchenette.