I would advisethat you contact Airbnb and explain the situation which you can back up if neccessary with documents or mails from the architect. They will help you by finding alternative accomodation for your guests, propose that you give a full refund , minus airbnb fees and hopefully avoid the automated posting on your profile saying you cancelled a guest which is never good for people booking their holidays, especially with a host with only one review. You may get your calendar blocked but this not important as the work on the house will be going on and you will not be eligible for super host status, but this is a long way off.
It's to everyone's benefit that Airbnb sort this out for you. You have a luxury listing in an idyllic spot which will be very popular when it's finished to the high quality you are aiming for. The photos are wonderfull and any guests are very lucky to be able to stay there. To add sugar to the pill of disappointment your guests will feel, you could offer a special discount on another stay with you at a different time.