The £369 million renovations of Buckingham Palace have been well managed but structural damage and the discovery of asbestos, which led to cost increases, “could have been foreseen”, a report has said.

The National Audit Office (NAO) said the 10-year project’s net expenditure at the end of March was £238.9 million – 65% of the budget.

On Tuesday, the NAO’s report said work on the East Wing came in at 78% over its estimated cost – with a water leak detection system and two lifts installed as well as new en-suite accommodation and five additional guest lavatories.

The report said 82% of operational improvements are complete, and the palace has made “good progress against the overall programme objective
to mitigate the real risk of operational failure”.

Completed steps include moving water tanks, installing new boilers, a switch panel and back-up generators, removing 3.5km of dangerous electrical cabling, the work on the East Wing, and some projects in the West Wing, the NAO said.

According to the progress report, the East Wing works, completed in October last year, found “more asbestos in the floor voids than anticipated” and the discovery of structural damage meant a redesign was required.

The NAO said in September 2023 the palace’s in-house programme management office (PMO) calculated the impact of the asbestos discovery, removal and change in strategy was £10.1 million.

Asbestos has also been discovered in other areas too, affecting other projects, the report added.

The NAO said the total capital cost of repairing the damage to joists and jack arches by the end of March was £3.1 million, not including costs incurred through delays or design fees.

According to the report, the PMO switched to an alternative design team for the completion of the West Wing to “mitigate against further delays”.

Detailed planning began in 2017 and in 2018 some 3,000 Royal Collection items including priceless paintings and decorative works of art were moved to allow the work to begin.

Essential works, which are being funded through an uplift to the Sovereign Grant, were required to protect the building from the risk of fire or flood by upgrading its infrastructure and to improve access.

The hub of the heating system was completely replaced in 2020 to improve the Palace’s energy efficiency, and step-free access to the front of the building was created in 2021.

The grand entrance, the grand staircase, the marble hall and the picture gallery were resurfaced in time for the late Queen’s Platinum Jubilee celebrations in 2022, with the picture gallery roof also replaced.

The report, penned by NAO director Lee-Anne Murray, said delays to the project meant work on the palace’s South Wing will now run alongside work to the North Wing, adding there was “a risk that there is not enough capacity in the PMO to manage this”.

It added: “In addition, as the programme comes to an end, key staff may leave.”

The report added: “While risks remain to the overall value for money of the programme, particularly given the backloading of work on to the last two years and the limited remaining unallocated contingency, if the Household continues to manage risks effectively, its approach to the programme should set it up to deliver good value for money.”

Head of the NAO Gareth Davies said: “Updating decades-old plumbing, heating and electrics, as well as adding new lifts and lavatories in one of the UK’s most famous buildings, is a significant undertaking which has been well-handled to date.

“Sound planning and project management has enabled the programme to remain within budget to date and respond well to challenges such as the pandemic.

“While risks remain as the programme draws to a close, if these continue to be managed effectively, the result should represent good value for money.”

The report said the PMO estimated that 11% of the total budget – £40.8 million – will be spent on management costs.

The project will receive £100 million of funding, or 27% of its budget, in its last two years and expects to spend £51.5 million in the final year of the programme.

Buckingham Palace – seen as monarchy HQ – has served as the official London residence of the UK’s kings and queens since 1837.

It has 775 rooms, including 19 state rooms, 52 royal and guest bedrooms, 188 staff bedrooms, 92 offices and 78 bathrooms.

The project, which has been under way for seven years, is entering its final three years.