Ha, it was actually their website this came from! Is that a coincidence - I'd never heard of them, are they good?
I am not sure their online quote tool can handle these kind of bespoke things but I will give them a call. Probably not until Monday, anyone got any guesses what it might cost?
This is a bigger departure from the design than might be ideal in the timeframe - arrival in 2 weeks, construction the week after - though it is probably the ideal in finished look.
We talked earlier about simply getting a big support beam in wall 3, keeping that wall the same otherwise and then getting rid of B-D. That company will sell these fancy beams for a big enough span:
https://www.donaldsontimberengineering.co.uk/products/joists/open-web-joists/
