@Kevin Jorgensen My thoughts are that you are wading into very subjective waters. Nobody on this forum can give you the correct answer without knowing more about your home.
Your assumption about staying under $225 a sqft sounds a lot like the sellers I meet who say "If that 2,000 sqft house sold for $200,000 then my 3,000 sqft house should sell for $300,000." -Ahhh if only it was that simple!
There's a least a dozen questions to ask which will affect the ROI of this investment:
Average sqft in your neighborhood versus your sqft? Neighborhood is 2500 and you're 2000? Okay. Neighborood 2500 and you're already at 3000. Bad idea.
Can you make the addition flow into the current floor plan? Or would it just feel like someone added a giant box onto an existing house?
What are you adding? A 4th bdrm? a family room or a basketball court? Not much ROI on the basketball court. A large master suite or a family room open to kitchen should pay back nicely though.
The devil's in the details of an addition. You can look at building costs per sqft all you want. But what matters is how much your addition will cost. Will it require an upgrade to the entire HVAC system and electrical panel? That's a $10,000 question right there.
How much more will the house rent for? If the long term goal is to rent it anyway, will the additional make financial sense for the rental value?
Is the rest of your house small & dated? Then the new big beautiful addition won't have much value. However if the addition is the cherry on top, the ROI will be better.
My shoot from the hip number is if you spend about $125 per sqft addition you probably won't lose money. If the cost is higher, you should really investigate the questions above.