The site has been rebuilt in recent years to provide state-of-the-art facilities, with the final stage completed in 2010, thanks to fundraising by the public.
But hospice chairman John Guille said problems had been found with the structure.
‘The remedial work is too extensive for the hospice to operate in a normal way, which is why we need temporarily to move to the PEH for this work to be undertaken.
‘For some time there have been some signs of dampness in the ground walls of the hospice.
‘Initially this was considered part of the drying out process of the new building.’
Hundreds of damp readings have been taken from around the building and specialists have been investigating the structure.
‘The conclusion is that parts of the building are damp to an extent that is beyond what is normal in a new structure,’ he said.
Further tests have been carried out on the water and waste systems, as well as the underfloor heating.
The building has a large network of pipes to cover the en-suite facilities, additional bathrooms, handwash stations and kitchen.
Discussions with the hospice insurers and loss adjusters have taken place, but that process was delayed by Covid-19.
It is hoped that work will start in a few weeks and be completed in early 2022.
While patients will move to the PEH, support staff will continue to operate from the St Andrew’s site.
Hospice director Jo Boyd said it was expecting to move into Le Marchant ward on 18 and 19 May, with about two or three hospice residents needing to be transferred.
The ward has been divided up into single-bed suites.
‘The hospital staff have been absolutely amazing and helpful,’ she said.
‘We have seven beds at the hospital, and we are normally at about 80% occupancy, so I’m hoping we won’t notice too much the reduction in beds.’
The expected cost of the work and who will be paying for it is not being revealed at this stage.
Mrs Boyd said the hospice would continue to need support from the public.