Brown long-eared bats have been discovered in old buildings due to be demolished in Shenley.

Derelict buildings are being knocked down to make way for a new £12 million mental health facility on the Harperbury Hospital site, in Harper Lane.

But builders discovered a pair of brown long-eared bats roosting in the roof and common pipistrelle bats were also seen during a site survey. With the help of ecological consultants Naturally Wild, Hertfordshire Partnership NHS Foundation Trust set up bat boxes in 15 locations around the site last week.

Senior ecologist Frankie Lowe said: “The brown long-eared bats were spotted emerging from a roof during an evening survey we did earlier in the year. As the roosts of these animals are protected, we needed to apply for a licence on behalf of the trust.

“We have put measures in place to ensure the bats will be protected from harm, by providing new homes for them and carefully dismantling the roofs of the old buildings by hand to check for bats.”

The new psychiatric intensive care unit (PICU) and rehabilitation ward will be replacing inpatient facilities usually based at St Albans City Hospital and is due to be completed by June.

There will be 30 bedrooms in the two units, all with en suite bathroom facilities and individually controlled heating and air conditioning. Both units will have a courtyard with plants and environmentally friendly gardens for patients to use.

The discovery of the bats has not delayed building work. Long-eared bats have ears which can be as long as three quarters of their body length. The pipistrelle bat is the UK’s most common species of bat and can weigh as little as 3g.

Both species are found in low numbers across Hertfordshire, and brown long-eared bats are rarer than pipistrelles. Originally both types would have made their homes in trees, but roofs of houses and other buildings often make safer homes. All species of British bats are protected by law.