Premier House, the official house kept for the prime minister, is in a dishevelled state partly as a result of the multiple purposes it serves, Heritage New Zealand says.
The historic Wellington building on Tinakori Road in Thorndon, also known by the title The Prime Minister's Residence, is about a 15-minute walk from Parliament.
Prime Minister Christopher Luxon says he is not living there because it needs significant maintenance work.
His choice came under scrutiny on Friday, when it was revealed Luxon was receiving more than $50,000 a year as an accommodation payment while living in his own Wellington apartment.
Luxon later pledged to pay back the amount already paid to him.
Heritage NZ Pouhere Taonga confirmed it is working with other agencies to find solutions for a badly needed upgrade to the building.
Regional director Jamie Jacobs said Premier House is also used to provide guest accommodation for visiting dignitaries and as a space for entertaining.
The site is managed by the Department of Internal Affairs. The Premier House Board gives advice on the long-term stewardship of the building as a national heritage asset but does not have any decision-making authority, Dr Jacobs says.
Parts of the Premier House property date back to 1843. It was purchased to become the prime minister's residence in 1865, when the capital was moved to Wellington and underwent significant expansion when Prime Minister Sir Julius Vogel lived there in the 1870s.
It became a Category 1 historic place in 1988, and has been scheduled as a heritage place in the Wellington City District Plan.
"A Category 1 listing indicates that the place has outstanding significance to the nation," Dr Jacobs says.
Advice about the site provided by Heritage NZ in recent years includes expertise to help make decisions about security and an upgrade to the cottage portion.
But there are important decisions ahead "to firmly establish the identity and function of Premier House", Jacobs says.
"And ... to find pragmatic solutions for a much-needed upgrade to the building that also continue to uphold its high level of significance to the nation."
What is Premier House like?
Dr Jacobs describes the building as architecturally "idiosyncratic", due to the mix and match additions made to it throughout its life.
It has "a southern wing built circa 1862 as a typical colonial cottage; a major neoclassical - Italianate addition, circa 1875; further alterations in 1926 and 1935 that introduced bungaloid features such as sleeping porches.
"And a top to bottom refurbishment circa 1990, with some postmodern features - the last phase of substantial work."
Premier House currently has a Government Valuation of $37 million and among its modern additions is a water meter.
But aside from the patchwork nature of Premier House, Jacobs says it also had an "unsettled identity" because of the range of roles it serves:
"Is it the PM's residence? A guest house for visiting dignitaries? A place for high level entertaining?"
The site, with its manicured gardens, is also "the only Town Acre of the original colonial plan of Wellington that has not been subdivided."
"The possibilities for further developing the gardens would be a great amenity for the city," he says.