Building a house

Has anyone done this? Are you able to offer advice?
I would like to have a house built for myself. It would be quite small, very well insulated and in a good position for access to shops and transport etc.

I have begun the process but am still at an early stage. I have a plot and have an architect who has drawn up plans.
We have submitted these for planning consent and after a few hiccups hope they might be accepted soon.

I would need to sell my house to pay for the building and I expect the process won't be straightforward. I will need to live somewhere while the new house is being built.
The architect suggests it could take a year. Is he being optimistic?

My DC think I am bonkers and are sure the project will be far more expensive than I imagine, take a very long time and will be incredibly stressful. They are probably right but I would like to pursue it as far as I can.

I'm mid 70s now, on my own and I would like to have a few good years to enjoy the dream!