I have a single story house with around 1300 sqft. I want to extend my house in the back 20ft for the entire width of the house. I would also like to add a second story on top of that 20ft extension.

The second story extension is coming out expensive at this time, so I would like to do that maybe like 2-3 years down the line once i have the money.

What is the best way to plan this so there is minimum amount of overlap in work and money?

For example, Is it better to consider the future extension and build a stronger foundation at this time for the single story extension? Any other suggestion?