Yes this is enough space.
You will likely be able to build bigger with planning permission , the planning fee is similar to PD, the amount of paperwork is comparable. Planning permission means a short delay (which you may need to find a builder).
Don’t forget the small print of PD that says you can’t build too close to a boundary.
You should plan for it to likely cost £2000 per square metre meaning what you are proposing could be £150,000 by the time you have bought kitchens, bathrooms, carpets etc

If you post a rough plan of your house you will likely get some creative ideas for layout. Don’t forget to note where the garden and parking are

Thank you Jonathan, that's really helpful, especially the costing perpsective too. I'll add some pictures and a rough floorplan (hand drawn) when i get home this evening).

When you mention PD fees, do you mean the certificate of proposed lawful development cost? If the extension is PD then we wouldn't necessarily require the certificate.

Is anyone aware of space standards for older people accommodation? AI has brought up conflicting measurements and regs which is all quite confusing. Its also not necessary to be wheelchair friendly, but I thought it made sense to build to that standard and future proof.

I agree you should future proof the space.
It makes zero sense to not get a PD certificate of lawfulness because if you are spending alot of money you need to know you can sell it later and recoup the money. If you can’t prove at the time that you sell that the extension was built lawfully you will have to get the certificate then (accepting that PD may not still be around in future years in which case you might then need planning permission). It’s just easier and possibly cheaper to do it now

Great point! Thank you !