Mill houses have long been sought-after properties, offering a heady mix of history and beautiful waterfront settings.
An additional attraction for some house-hunters is that modern laws restricting construction in flood plains mean many have no close neighbours and often boast large grounds.
The golden age of watermills in France stretched from the 12th to the 19th Century, when first steam engines and then electric power helped grind corn to flour, stamp mineral rock to powder, operate forge bellows or turn machinery for everything from looms to stirrers in paper-making factories.
Loss of old mills
However, laws on returning rivers to their ‘natural’ state mean that many mill races and the weirs and dams which supply them are now threatened with removal.
The legislation dates from 2010 and was reinforced in the 2021 ‘Climate and Resilience Law’.
It is estimated that some 7,000 mill races have been destroyed as a result of the measure.
“Personally, I think the law is an absolute aberration,” says Stéphane Viennet, managing director of hydroelectric consultancy Hydreole.
“Many of the destroyed mills have been in place since medieval times and throughout most of their lives the rivers which supply them have been full of fish.
“It is only since farmers have been using pesticides and synthetic fertilisers on a large scale in the last 70 years, and we have had more frequent heatwaves, that there has been a problem.”
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Stéphane Viennet (centre) and other members if Hydreole's team in front of a Kaplan turbineHydreole
Added to the pressure from the state, estate agents such as Patrice Besse, who specialise in chateaux and other historic buildings, say they are getting more queries about mills which can be converted to hydroelectric power from people who simply like the idea of being self-sufficient.
However, Mr Viennet warns that this is not as straightforward as many people imagine.
“Water mills, which turn at around 30 revolutions a minute, are usually not suitable for electricity generators, which need at least 200 revolutions per minute to operate,” he says.
“We have been involved with some electricity plants that use a mill wheel, but a better solution is usually a turbine.”
Not all old mills are suitable for turbines, which usually require a ‘head’ of water of around two metres.
Most mills converted to hydroelectric power use a Kaplan turbine, invented by Czech engineer Viktor Kaplan in the early 20th Century.
“We have just finished refurbishing one mill which was converted into a hydroelectric plant 70 years ago, and the turbine needed hardly any work at all,” says Mr Viennet.
“Modern ones have variable pitch blades and electronic controls, which make them more efficient and mean they can adjust better to the amount of water flow.”
The cost to convert a mill to hydroelectric power varies considerably.
“Most people will spend at least €50,000 to install a power station,” says Mr Viennet.
“We do not generally advise people to put in plants that will take longer than 12 years to pay back the investment, which is longer than some solar panel generator schemes.
“However, an advantage is that the value of a mill with an operating hydroelectric power plant is much higher than one without.”
Mr Viennet says that Enedis, which operates France’s electricity distribution network, usually has no problem connecting hydroelectric plants, making it possible to sell surplus power.
However, there are no guaranteed tariffs to sell this, unlike for solar power, so the price can vary.
Banks may partially fund hydroelectric conversions with loans, as long as they are presented with a full breakdown of costs and future earnings from energy experts.
Restored mills are often at the higher end of the property market: estate agency Barnes International lists one in Eure-et-Loir for €2.4million. It boasts six bedrooms and three hectares of land.
The mill machinery is still in place and on display, with potential to convert it to hydroelectric power production, says the agency.
