Thousands of homes in some of Britain’s most expensive areas will be designated as facing a high risk of flooding for the first time from next month. This could lead to a sharp increase in insurance costs and even a fall in the value of their properties.

From August, the Environment Agency (EA) is introducing maps to take into account flooding caused by heavy rainfall for the first time. The maps currently only factor in coastal and river floods and show 517,000 homes at high risk.

That number could now double to 1m, according to JBA Consulting, which drew the new maps, and will include homes in areas such as Kensington & Chelsea and Hammersmith & Fulham in west London, Salford and Trafford in Manchester and parts of Reading, Berkshire.

Around two-thirds of the damage caused by last summer’s deluge, which left 13 dead and 44,600 homes flooded, resulted from surface water unable to drain away.

The likelihood of flash floods is expected to increase with climate change. Winter rainfall is predicted to go up by 10%-30%, while rainfall intensity, where there is a heavy burst of rain overwhelming drains, could increase by up to 20% by 2080, according to government figures.

Norwich Union (NU), Britain’s largest insurer, has been working alongside JBA and is expected to use the maps to calculate future premiums. If Norwich Union’s own claims data shows that properties on the new map have been flooded in the past because of heavy rainfall, households may already be paying higher premiums.

However, an estimated 450,000 of the 500,000 extra homes at risk may never have been flooded before, according to JBA. While their insurers will continue to cover them under a moratorium agreed with the government in 2002, their premiums could go up by as much as 20%, or their excesses — the amount of a claim you must pay yourself — could jump by as much as £1,000-£2,000.

Simon Black, head of flood mapping at NU, said : “Existing customers could also see their premiums increase if our understanding of the risk to a particular property shows a higher chance of flooding.”

Other insurance firms in touch with JBA include Royal & Sun Alliance. It said: “There is a major gap in our understanding of surface-water flooding. Monsoon-type rains which we saw last year are likely to occur more frequently.”

The new data could also hit home values. Mike Ockenden, director of the Association of Home Information Pack Providers, said: “The new maps should help people identify the problem so they can remedy it. This can cost a few thousand pounds, and I expect this cost to be factored in to the value of the property.”

He expected flood-risk information to be included in Hips by the beginning of next year.

Alice Stuart-Menteth at Risk Management Solutions (RMS), which contributed to the Pitt Review, said: “The EA’s maps only tell part of the story. When you take into account surface-water flooding, the picture becomes more alarming because thousands more homes are exposed than the government suggests.”

For example, the current EA map for Kingston-upon-Hull, which experienced extensive flash flooding last June, categorises many of the areas as just being at risk of coastal flooding.

Peter Ainsworth, the Conservative environment spokesman, said: “There have been concerns that the EA mapping hasn’t always reflected the experience of local people . . . while nobody wants to discover that they’re living in a high flood-risk area, it's vitally important we have accurate up-to-date information.”

Some 124 towns and cities have been mapped by JBA. Laser-mapping technology has allowed accuracy to within 15cm so individual streets and houses can be identified. The maps will predict where water flows and accumulates in heavy rain.

Separate analysis by RMS shows areas in Manchester are at risk of floods because of inadequate drainage. In London, Southwark, Battersea, parts of Hammersmith & Fulham and Kensington are at risk.

RMS said: “It is important people know what type of flood risk they are exposed to. If you think you are only at risk from coastal floods, you may not be as vigilant when the Met Office issues heavy-rain forecasts.”

Research by Landmark, which records flood claims in areas not covered by EA maps, also shows many affluent neighbourhoods affected by inadequate drainage. It classifies areas with more than five flood claims per 1,000 residents as high risk. These include the London postcodes W11 to W14 — between Shepherds Bush and Notting Hill where a typical three-bedroom property can sell for £1.4m — and RG2 8, southwest of Reading between Whitley and the M4.

At present, homeowners living in high-risk areas are given some measure of protection under a 2002 agreement between the insurers’ trade body, the Association of British Insurers (ABI), and the government.

Homeowners can normally rely on cover if they stay with their existing insurer, but they face inflated premiums and high excesses — sometimes as much as £10,000. The “memorandum of understanding” only specifies the need for adequate investment in coastal and river flooding, but there are talks to include drainage as a key factor in extending the agreement.

The ABI said: “We constantly review the memorandum. We hope that there will now be more research into surfacewater flooding as well as investment into improved drainage.”

Following the Pitt Review, the government pledged at least £5m for the development of surface-water management plans in the highest-priority areas.

The new data will not be available to the public, and insurers will have to purchase the maps independently. This may change when Hips include flood-risk information.

It is difficult to find out if your house is at risk of so-called “pluvial” flooding from drains, although you could ring your local council for advice. You can also check the number of flood claims in your area using Landmark’s site, homecheck.co.uk .

If a mainstream insurer refuses to cover you, or hikes your excess to unmanageable levels, it may be worth approaching a specialist such as Adrian Flux, of Delite Insurance.

You could also cut premiums by 20% by adopting some preventative measures. These include investing in sandbags or fitting non-returnable valves on your drains to stop sewage surges.

Don Farrell, managing director of Aquatite Systems, which supplies flood defences for homes, said it costs about £1,000 to protect the average house without a garage.

You can call Floodline on 0845 988 1188 or visit environment-agency.gov.uk. For home protection, visit floodforum.org.uk .

How to find new maps

The new Environment Agency map will for the first time tell insurers which homes are at risk of pluvial flooding from heavy rainfall. At the moment, EA maps show only the risk from fluvial (river or coastal) flooding. You can view the current maps at environment-agency.gov.uk/subjects/flood/ .

The new EA maps will not be made public, but you can get more information on pluvial flooding from Landmark at homecheck.co.uk . It assesses the risk of flooding using numerous sources including claims data.

We fear drain threat will send our premiums up

Phil Garland, 35, a construction surveyor from North Cave, near Hull, was flooded last summer due to an overflowing stream and poor drainage. While funding has been allocated to protect against the stream, he says there are currently no plans to improve the drains.

His property is designated as at risk of flooding from rivers but not drains and he hopes the new Environment Agency maps will galvanise the local authority to act. He said: ‘There have been some close calls in the past few months. Water along certain lanes has been about a foot deep.’

Garland, who lives with wife Liz, 31, and their twins Max and Elliot, aged three, has been in touch with David Davis, the former MP for the area, to lobby the agency and Yorkshire Water.

His Barclays home insurance is up for renewal next month and he expects it to increase. A neighbour had his flood claims excess increase to £8,000 recently.