I wonder if anyone can put me on the right track about having a house built here.
About what would it cost to build a two storey house of around 60-70 square metres? Alternatively, a single story of about 90-100?

These figures are very speculative, and may well be different eventually, but just to get a ballpark amount for the building cost.

Quick background -
Currently we rent a two-storey semi-detached house (baan faet) and the plot of which is about 82 sq metres and the house about 45-ish. It was fine for just the two of us when we moved in almost 13 years ago but we have expanded over time.
It’s about 3 kms from the Mall, Ngam Wong Wan, Nonthaburi and we love it here. Very handy for almost everything and I can be in Bangkok on the MRT in about 45 minutes should I need to.

However, the wife and her daughter are trying to get a younger sister to give them about 300-400 sq metres of empty land she owns so that they can build a house. This is in Banokville in northwest Pathum Thani, very close to the border with Nonthaburi and Ayutthaya, so extremely rural.

Basically, I don’t have enough to pay for building a house so the wife’s daughter, 30 and unmarried with a decent job, would have to borrow the money necessary and I would use the money I pay for rent here to help with the repayments. But this is Thailand …

So, some advice please. Is the house area I mention above about right for two people and the daughter visiting at weekends?

I suppose 3 bedrooms, plus a room of about 3.5 x 5 metres for a man-cave/study/hide from the family room, 2 bathrooms, 1 decent sized living room and a biggish kitchen. A carport is OK because I don’t run a car (just a couple of motorbikes) and also we'd need a laundry/utility area at the back.

What would that work out approximately given the rural nature of area? And is it possible to purchase ready-made house plans from an architect or similar?

This is all very much beginning thoughts, but any input to help concentrate the mind is much appreciated.