The average house in London now costs more than £300,000, says the Halifax.
N.Ireland is now second only to London and the S.East
The UK's biggest mortgage lender says the average London house price rose by 4.9% in the three months to June, to £313,122.
It says this means the capital's average house price is now above the threshold for inheritance tax, currently at £300,000.
House prices are rising fastest in Northern Ireland, up 8.5% in the same three months and 47% in the past year.
Halifax economist Tim Crawford said that as well as London breaching the inheritance tax threshold, the recent rises had pushed average prices for the South East above the £250,000 watermark for 3% stamp duty.
"The typical homebuyer in London and the South East, in particular, faces a rising tax burden due to the government's continuing failure to increase the inheritance tax and stamp duty thresholds in line with house price inflation," he said.
The boom in prices in Northern Ireland has been dramatic in the past two years.
With average prices of £229,000, the region is now the most expensive in the UK outside London and the South East.
Two years ago, it was the second-cheapest region in the UK.
One result of this is that eight towns in Northern Ireland top the list of towns where prices have risen fastest.
The cost of buying a flat or house in Newtonards has shot up by 64% in the past year - to £228,000.
It has been followed by hefty price increases in Craigavon, Newtonabbey, Downpatrick, Carrickfergus, Belfast, Lisburn and Newry.
The Halifax said this regional boom was due to a buoyant economy, immigration and strong demand from second-home owners.