Can I do some sort of an addendum to rectify this mistake?

As 4 homes are being constructed we project no more than 6 months...worst case scenario maybe 8 months.

I'd say +/- 5% of the sales price between best case and worst case. 

I've made the same mistake myself many years ago.

We don't have many providers of TK here and from what I see they will often put you in the worst parts of the city. 

Try and learn from this - what mistakes were made and what was overlooked, so you don't repeat them.

Get the experience, make small mistakes that you can recover from and build a track record then you're ready to tackle these large complex deals.If this deal were to go pear shaped and it could for so many reasons that would be outside of your control, it would a huge set back financially and emotionally.If it was a good NNN like a Chase, Cvs or Walgreens deal buyers would be bidding it up to a 5 cap.

I Agree, pay the extension fee and note where you went wrong in your timeline of construction so you can help not repeat the mistake.

If they cannot tell you how much they have in reserves then don't buy into their building (I made this mistake and only found out later that the state of the association was a total mess).

Learn and surround yourself with people that have made mistakes, so you don't have too .