A "surprised and disappointed" couple took their son to court after learning he'd double-crossed them in a property purchase and they didn't actually own the house they called a home.

Amin and Usha Siddiqui immigrated to New Zealand in 1991 with their son Ashish and daughter Supriya.

While the couple did well on arrival, Ashish thrived and by 2006 had bought his own home while his parents continued to rent.

An arrangement between Ashish and his parents allowed them to buy a house in Auckland in May 2006 for $249,000.

Ashish arranged, through the realtor and bank, for the property purchase to be fully funded by mortgage finance but he provided collateral security to meet the lending requirements.

Amin was told his name was on the title by Ashish but in fact it was registered to him and his wife Yashika.

Believing the Manurewa house was theirs the couple paid the mortgage, through their son, along with all other expenses including ground rates, water rates, insurance as well as maintaining the property.

Two years later a family feud erupted resulting in a lengthy litigation process to gain legal ownership of the house after Amin and Usha discovered they were not on the title.

Senior relationship property lawyer Jeremy Sutton said the case, which he was not involved with, was a prime example of why independent legal advice should be sought by both parties in any financial agreement, even if it was between family members.

"They go wrong a lot," Sutton said.

Jeremy Sutton, senior relationship property manager, says all financial agreements need to be in writing even if they're between family members, to protect the lender. Photo / Supplied

"They need a loan agreement drawn up by a lawyer to make sure that the interests of the person lending the money is protected."

Sutton said in a perfect world people wouldn't have to resort to getting lawyers to put everything in writing but relationships changed with time resulting in disputes over the intentions.

"Because of that people get into these sort of issues.

"When that initial transaction is done, when they are buying the house, it is the time to do all of the documentation."

High Court Justice Paul Davison ruled last year that the house was to be held by Ashish and Yashika in a common intention constructive trust for the parents, as that was the intention of the purchase.

This essentially means the property does belong to the parents.

On the day the couple moved into the house they were greeted with a letter from Ashish which began My darling Mum + Dad, Congratulations!! Welcome to your new home.

But, he later claimed in court the money paid to him by his parents was just rent.

Davison, however preferred the parents' evidence over Ashish's and rejected his claim for a 13 per cent interest in the equity but allowed adjustments for his contribution.

Ashish and Yashika refused to relinquish ownership and appealed against the decision, claiming the trust should have been based on reasonable expectations which would have given them an equity in the property.

They also challenged the adjustments, seeking a guarantee fee for providing collateral, compound interest on other payments they were granted and disputing the award of costs to the parents, Davison made.

Amin and Usha cross-appealed the adjustments focusing on the amount awarded to their son for bathroom renovations and interest.

Judge John Kós said in an appeal decision released this week the court would be wary of contradicting Davison's view as he was an experienced judge who had the benefit of seeing and hearing the witnesses during the trial.

Greg Stringer, appearing for Ashish and Yashika during the appeal, conceded imposing a trust was inevitable but argued for a reasonable expectations trust to preserve a proportion of the equity for them.

Stringer stressed the parents had stopped making mortgage contributions in April 2015 which he said was inconsistent conduct from a party alleging the property was theirs.

Judge Kós was not persuaded by the submissions and the non-payment of the mortgage was explained by the dispute resulting from the denial of the parents' ownership.

He was satisfied the circumstances required a constructive trust to be imposed.

An $18,500 adjustment awarded to the son for bathroom renovations was reduced to $8460 due to unsatisfactory oral evidence, given by both sides, on who paid for what.

Interest on mortgage payments, $91,188, made by the son and wife as well as the bathroom expenditure, $8460, were awarded while the parents would receive interest on mortgage overpayments of $25,879.

Judge Kós also upheld the costs award made to the parents.

Sutton described the case as a reversal of the usual situation where parents gave money to children, where the presumption was it was a gift not a loan.

"It's up to the parents then to get documentation to prove it's a loan and not a gift."

He said there were numerous ways to protect your investment including a Deed of Acknowledgement of Debt, setting up a family trust, having your name on the title or having a prenuptial agreement which sets out the obligations and expectations of each party.

- Leighton Keith, Open Justice