So 300sq feet approximately which is a utility room, so it's a fair bit of space.
I'm only suggesting this, but as we know the market is a bit stupid now and trying to get another property would be another job .
If the house that you thought you were getting isn't going to be built until next year its probably going to be more expensive so I would sit down and think about what you have and what you might have and the differences between everything. Also look at the positives as to why you chose the house, location, price, etc.
It's not ideal what has happened to you and I'm no fan of developers but walking away and restarting the search for a home is adding more stress.
This happened to us about 25years ago 1998, and I was furious for a long time, still bought the house and added a few bits , sold it in 2001 for 3 times we paid...
Theres a lot of non financial considerations to take into account and as others have said legally you don't have any recourse, the developer won't care and probably sell it at a higher price.
Again I'm only suggesting this and you need to make the decision.