What could be Wanaka’s biggest house — a $20 million mostly subterranean home called The Sanctuary — has been designed to replace a mansion on a prime Wanaka site.

The Nature Preservation Trust’s proposal to bulldoze a two-storey mansion built for aviation identity Ray Hanna and replace it with something three times bigger is causing concerns for three neighbours.

Sorted Architecture principal Steve Humpherson designed both the 650sq m house destined for destruction, built for about $1 million in 1999, and the proposed 2008sq m replacement.

The new house is opposed by the Upper Clutha Environmental Society and neighbours Whare Kea Lodge, Garth and Andrea Hogan and Andrew and Joanne Todd .

In written submissions to the Queenstown Lakes District Council they say the new house and related earthworks are inappropriate for the outstanding natural landscape.

UCES spokesman Julian Haworth said significantly adverse effects could be avoided if the owner just lived in the Hanna house.

The trust’s sole trustee is Wanaka lawyer Janice Hughes, who cannot release her client’s name to the The News.

It is understood the family, formerly from overseas, are New Zealand residents.

The new house would be 68% underground. The trust’s vision is to restore the habitat on the 7.6ha site. About 4000 trees have been planted.

The Hanna house was one of the first mansions along what has become a sort of ‘‘embassy row’’ on the western edge of town.

It was built in the Central Otago vernacular of schist and cedar shingle cladding, Karaka coloursteel roof and recycled bridge beams.

Wanaka’s western boundary is governed by restrictive planning rules. Houses must be reasonably difficult to see.

Mr Hanna got non-notified consent, under the district plan rules in force in the late 1990s.

‘‘It was a great project. It got me started on designing with Central Otago stone and big old bridge beams. I have done a few of those now,’’ Mr Humpherson told the The News.

The house sold several times after Mr Hanna died in 2005.

The new house would have six bedrooms, two kitchens, multiple lounges, a gym, a library, an elevator and an 18›m long swimming pool. There would also be a 449sq m implement shed.

Mr Humpherson agreed the size is ‘‘way up there for Wanaka’’ but could not confirm if 2008sq m would be Wanaka’s biggest.

Planning documents reveal a 1194sq m house has been built on Roy’s Peninsula.

Mr Humpherson estimated the new house could cost about $10,000 per square metre to build, because of the size, quality of finish, joinery details and mechanics involved.

He’s never designed a house quite like it.

The upper floor cladding includes custom glass and reinforced concrete facade panels with a 3D face to the southern side, creating shadows to soften the visual effect.

The above ground, visual component would be just about half of what can been seen of the Hanna house.

Five ‘‘cave›like portals’’ would lead to underground areas.

Landscaping, planting, green roofs and green walls would make the new house much harder to see than the Hanna house, Mr Humpherson said.

It would need a lot of earthworks — 7600cu m, over 17,000sq m.

‘‘It’s been a great experience. The client has been pretty hands on too . . .He didn’t ask me for a 2000sq m home. That’s just evolved from our discussions.’’

It is the second consent application for the trust.

Reasonably hard to see . . . an artist’s render of the visible portion of a new house that has been proposed to replace the Hanna House on the western boundary of Wanaka. IMAGE: SUPPLIED/SORTED ARCHITECTURE

In 2019 the council granted a non›notified consent.

But Trilane Industries Ltd, the owner of Whare Kea Lodge, was concerned about significant effects during construction and sought a High Court judicial review.

Judge Rachel Dunningham ruled the council should have considered both temporary and permanent adverse visual effects. The consent was declared null and void.

The plans and earthworks were amended and the trust reapplied for non›notified consent in June 2021.

However, in November, independent commissioner Wendy Baker decided it should be notified.

Trilane still has concerns about significant effects and has opposed again.

The Hogans and Todds say the Hanna house fits the landscape better, because of recessive colours and materials.

Mr Humpherson was unfazed about reaction to his designs.

‘‘You are always going to get critics. It is good. Otherwise, life would be quite boring,’’ Mr Humpherson said.