London Zoo’s aviary is among the most recent additions to Historic England’s Risk Register, with wider concerns over costs for at risk heritage sites thanks to the conservation deficit.

Overall, there are fewer entries on the 2016 Register (5,341) compared to 2015 (5,478) but fewer are economic to repair. According to Historic England, the conservation deficit – the difference in the cost of repair compared to the end value – has increased substantially, driven in part by a skills shortage affecting both consultants and tradespeople, with the average now £652,000 (US$797,000, €732,000).

The aviary is among a list of 11 other sites added to the register in 2016, including a 16th century shipwreck off the coast of Dunwich in Suffolk, Fort Purbrook in Portsmouth and Grade I-listed West Horsley Place in Surrey.

Designed by Lord Snowdon and built in 1965, the aviary at London Zoo was the first walk-through aviary in Britain. Now in need of repair after 50 years of service, London Zoo has secured £301,000 (US$368,000, £338,000) in Heritage Lottery funding to turn the aviary into a new space for both animals and visitors. An application for a full grant of £4.8m (US$5.9m, €5.4m) will be made following the outcome of the initial development stage.

"Many at risk sites lie decaying and neglected and the gap between the cost of repair and their end value is growing," said Duncan Wilson, chief executive of Historic England.

"The good news is this year there are fewer entries on the Heritage at Risk Register than last year. But as some places are rescued, others fall into disrepair."