I actually calculated this a while ago. I’m not going by just “essentials” but also taking in account that furniture will take up space. So you need just enough to not feel ‘cramped’.

Assume you need at least the following:

• Bedroom: (bed + cabinet for clothes): 8 m₂

• Dinner area (table and chairs) 10 m₂

• Toilet and washing basin 2 m₂

• Shower (including rack) 4 m₂

• Kitchen (sink, oven, hot plate, fridge, dishwasher, washing machine, rubbish bin) 8 m₂

• Living (couch, TV or whatever) 8 m₂

• Extra space[1] (your niche) 8 m₂

• Hallway (clothing rack) 6 m₂

• Optional [2] (outdoor) 10 m₂

[1] You could also distribute the extra space for the other rooms. Just consider it a sort of ‘backup’.

[2] For this I count a garage, garden, or bicycle storage. But I consider it optional since not everyone has or strictly needs those for good comfort.

Altogether, you then get about 54-64 sq m for an household of 1-2 adults (may include a small child or pet).

So a good fist rule might be 60 sq m, then add 20 sq m for each extra person.