Any advice on what size house to buy?

Are there any on going costs to your flat to cover general maintenance of the building such as shared areas, the roof and gardens? My cousin recently moved into a flat and I was quite shocked by how much he pays a month.

I guess it depends on how often you are going to use it and whether it will ever become your main home. If the former then probably ok. If the latter then think about what your future needs might be. A lovely problem to have. Good luck.

Are there any bigger flats available?
Would an extra bedroom be useful for grandchildren to sleep over for a treat, even though they live locally, as they get older?
Could you manage the maintenance of the house if you end up on your own (sorry if that's insensitive but maybe worth considering) or would you move to a flat at that point?
What an exciting problem to have!

I'm not sure what your plans are flopen you say this is a second home. If you are only spending periods of time there and its not your main residence then lock up and leave, a small home is ideal. If however you are spending a lot of time of there then opt for more space.

As in, the garden is just right (not too big, not too small). It's the flat that is compact.

The flat does have a nice garden. The issue is that it's small. 1 bedroom, 1 small living room, kitchen and bathroom. Is it worth spending more a big chunk more for another small bedroom and a dining room?

Are there any new build properties n the area?
It would have 10 years NHBC cover.

A flat without outside space is not a good investment, it’s really important to have some outdoor space even if it’s small.

Is there any 55+ housing available or housing trusts where you could rent?

For me the benefit of the outdoor space would be worth the extra money but that's just me.