I think it's fine.

If it is the only window on that wall of the house I suppose it would be okay.

If you change the smaller window, are there other windows on that same wall? It may be worse to have two different window patterns along the same wall, from the outside, even if they are different rooms inside.

I would want all the windows to match, but I don't know that it's super important.

it would be worth it, to me.

I am guessing you will be explaining it to everyone that visits, if you took the time to post it here for us to see.

Honestly. I am not sure a lot of people will notice.

I was involved in a project where they did window replacement and the windows were 6 over 6 with the exception of one window which was 9 over 9. (I was doing interior stuff)

It was in the bathroom by itself, but it was one of a whole row of windows along that wall.

I walked up the the house and I said "Why is One window different than all the others?" The contractor never noticed, the homeowners never noticed. They didn't even know which window I meant looking right at it. The homeowner said she was going to kill me because if I hadn't said anything she Never would have noticed.

The windows in the adjoining room are 3 x 9’s, so yes, definitely bite the bullet and fix it. It will always look a little off and bother you if you don’t. It’s nice of the builder to offer to split the cost at this point too. I also assume that the 3 x 9 looks better with the style of your house, since that’s what you originally intended.

What room is that? If it is the kitchen, then I would either leave it or change the single window.

Although I am biased. I dislike all those grilles so less is more IMO.

What does it look like from the outside?

Your contractor is really nice about that. Maybe you can reach out to the place ( they really should have caught this as well ) and see if they will give you a break on the price. Then sell the windows and split the proceeds between you and the contractor.

Sorry I dislike grids so the mistake works better for me than the 9/9

Same size windows get the same mullion pattern. You have not mentioned the style of your home. Like shutters (authentic), mullions belong on certain architectural styles and are a "must".

Colonial, Cape Cod, Victorian, etc. have mullions (patterns can be different). Craftsman windows, again, mullions a "must", in the particular pattern.

So the smaller windows still have mullions, style to fit the window.

This house is in Sandwich MA. Notice how the smaller window still has mullions.

Mullions had a purpose back in the day. There was no single pane of glass to fill a window. True divided, the mullions held the smaller pieces of glass in place. Now, of course, with the ability to make windows air tight, instead of true divided, there is simulated true divided.

You might enjoy reading about what you have and what was:

https://www.qualitywindowanddoorinc.com/blog/divided-light-windows/

For years my house had mullions on all windows except the sliding glass doors in Family Room. You are good with that opening.

$750.00 on a forever home? Will you be there 20 years? That's 37.50 per year to not look at windows and be sorry you screwed up the order but more sorry you didn't replace the mistake.

Thirty years? Thats 25.00 per year and in 2054 the money will be chump change.

I think the windows should be consistent in the house, especially in the same room. I'd pay the $750 to have the mistake fixed.

Looks like it's going to be a beautiful space!

Thank you all!

The large triple window in kitchen is the only window on that side of the house on that floor. If looking at the house from the back yard, to the left is a garage and to the right is our screened porch. Second floor windows all have 3x9.

As far as the single side window, thats the only window on that side of the house.

The front of the house has 4 windows on the first floor , and 7 on the second, all with 3x9.

Lastly, I believe our house is considered colonial

Yes, a colonial home would have 9 over 9 or 6 over 6, or even 6 over 1, if a colonial revival. 4 over 4 is more appropriate for a farmhouse.

I think it looks fine as is because the window is larger than the others.