The rise in prices will also nearly double revenues from inheritance tax as more properties push estates beyond the £325,000 threshold.
And the boom will grow, with Help to Buy boosting prices by as much as another 10 per cent, the OBR said. That will bring prices almost back to 2007 levels by 2018.
At the Budget in March, the OBR expected stamp duty to raise £7.7bn in 2013-14, but in fact it is raising £8.9bn as the market has taken off.
In 2014 it will raise £10.8bn, well above the £8.4bn earlier forecast.
And in 2017-18 it will raise £15.2bn, well in excess of the £11.7bn predicted back in March.
Part of the reason is fiscal drag, as the stamp duty brackets are holding still even as prices rise rapidly.
Analysis by tax firm Baker Tilly showed that if the thresholds increased in line with average house prices from 1997 to today the one per cent band would begin at £382,000, rather than £125,000.
And the three per cent bracket would kick it at £765,000 rather than the £250,000 where it actually takes effect.
Meanwhile the OBR showed a rise in revenues from inheritance tax. The levy will raise £5.6bn in 2018-19, up from £3.1bn last year.
Again this took the forecasters by surprise – March’s outlook said revenues were expected to rise to £4.1bn by 2017-18, which has now been hiked to £5.2bn.
The agency does not expect any slowdown in the market – it expects prices to rise five per cent next year and seven per cent the year after.
On top of that, while Help to Buy could raise prices the OBR does not expect it to ease housing supply.
“To the extent that Help to Buy lending is additional, our modelling suggests it would feed through roughly one-for-one into house prices in the short term, which we would then expect to feed through to a much smaller extent into house building,” said the OBR’s report.
Despite the worries the Help to Buy scheme received an extra boost yesterday as challenger banks Virgin Money and Aldermore joined the scheme. The pair will offer 95 per cent mortgages with a government guarantee taking some of the risk of the loans.