Um, I have a big issue with the way this story has been reported.
JK had the extension built in 2008.
It cost £50k.
Those parts seem to be widely agreed.
Her mortgage was extended, adding either £250 or £500/month to what she could claim as expenses (I suspect it was £250 as interest, and £250 as capital she couldn't repay).
So far, the taxpayer has therefore funded something like 12-18 months for the extension, therefore (12x£250 or 18x£250). Call it £3750.
Now, you may think that's wrong, but why do all the headlines say that taxpayers have "paid" (PAST TENSE) £50k? They have NOT paid £50k for her extension.
F*ing sensationalist headlines.
This whole thing is going to backfire very badly. Only rich people are going to go into politics in the future on the back of it. That will be a great thing - Not.