WWYD: Convert loft / move / BTL?

[weakestlink]

What would you do:

Currently have 3 bed 1 bath (& downstairs loo) house but DM needs to live with us / nearby. I would like to convert the loft or move but DH wants neither! He wants to buy a BTL one bed nearby... WWYD??

Option 1:
Extend loft to create 4/5 bedroom 2 bath house
Cost: £30k

Option 2:
Move to 4/5 bed house
Cost: £110k (meaning we'd double our current mortgage) research I've done seems to tell me our house is worth 180-185k but to get what we need (4 double/ 3 double 2 single bedrooms) would be in the region of £300k and possibly need work to bring up to spec :(

Option 3:
Buy 1 bed apartment
Cost: £90-100k
Buy and "rent" to DM who currently rents anyway so no problem paying and we could sell up eventually to get our money out.

Of course there is Option 4 which is that DM just rents somewhere nearby that you don't actually own. No outlay for that one. Of the ones presented, Option 3 sounds least problematic to me. If you circumstances changed for the worse you could always sell the 1 bed flat rather than have to move the entire family.

Of the three options you give, I think Option 3 - assuming your mother is still able to live independently. This is far less intrusive on your family life.

Option 4 - DM rents somewhere nearby is another good option though. Unless rents are too high.

We've had problems with her having to move several times over the past few years as landlords have sold up / moved in themselves / been repossessed! It's upsetting for her so we are looking for something more secure. She has good pension(s?) so doesn't qualify for any sort of social housing but can afford good rent but wouldn't get a mortgage of her own (67).

option 3 sounds like the no-brainer - with obvious considerations of access should she get less mobile.