ReutersPeople wait in line to visit an upside-down house.

You think Britain's property prices are insane right now? Well, they're about to go even more bananas by the end of the year.

The Royal Institution of Chartered Surveyors (RICS) just doubled its forecast for house price rises in 2015. It says that the price to buy a home will now rise by 6% by the end of the year, compared with its previous forecast of 3%.

It also thinks price rises are set to continue into next year, with the pace accelerating.

Essentially, the severe shortage of housing, coupled with the soaring demand from new buyers means that there is no other trajectory for property prices — the only way is up.

"While the UK housing market has seen some substantial volatility in demand over the last 18-months, the most consistent feature has been a distinct shortage of new instructions," said RICS Economist Michael Hanley. Instructions means people telling a property agent to sell their house — so, supply.

"With respondents reporting another fall in appraisals during August, and looking at general market conditions, we have no reason to believe this will change in the near term. Therefore, despite the fact that demand has been picking up in recent months, we have lowered our forecast for transactions for 2015 from 1.25 million to 1.2 million. Alongside this, we have revised our expectations for price gains this year up to 6%."

Just look at this crazy gulf between supply and demand:

RICS

Latest data from Britain's Office for National Statistics shows that the average house price in the UK is at £277,000 ($425,957). In London it is even more astronomical at £513,000 ($788,781).

New buyer inquiries increased for the fifth month in succession in August, according to RICS, but vendors looking to sell their homes "have yet to record any meaningful upturn since the middle of 2013, pushing average stock levels to record lows."

Simon Rubinsohn, RICS Chief Economist, says: "Given current market conditions, the latest data unsurprisingly shows house prices continuing to rise, and at an accelerating pace. As such, house price inflation has now quickened in each of the last seven months following a sustained period of easing towards the latter half of 2014."

"And there is good reason for this trend to be sustained into next year, however uncomfortable that may be for those looking to enter the market, given that so many of our members are telling us that they are struggling to replace the stock they have sold."