A Grand Designs eco-home built for £600,000 has hit the market with an eye-watering £3.25million price tag - but comes with a perk most homeowners can only dream of: zero heating bills.

Underhill House in the Cotswolds featured on the hit Channel 4 show when it was hailed as England's first certified Passivhaus property - meaning it is effectively air-tight and requires no central heating.

The six-bedroom property, dug into a hillside to be near-invisible from the surrounding countryside, was built by architect couple Helen and Chris Seymour-Smith in 2010.

The vast home sold for £1.4million in 2012 - more than double its construction cost - and has now returned to the market at £3.25million.

But while the asking price is enough to make most buyers gasp, the running costs are astonishingly low.

The house is designed to hold on to heat so efficiently that it needs no conventional heating.

There are no radiators, with warmth from the sun, appliances and people inside instead circulated through a heat recovery system known as an MVHR.

The current owner, Glenn Jones, described being 'phenomenally smug' in the ultra-efficient home because he had no bills to pay.

A university study into Passivhaus reported in 2014: 'Glenn said that their expenditure on electricity is nil, due to the solar panels on the roof.

'Other than that the only cost is £300 a year on logs for the fire and a new filter for the MVHR every six months.

'In a time of rising fuel costs Glenn remarked, 'I am feeling phenomenally snug'.

'It seems that although they did not set out to buy a Passivhaus, Glenn and his family are still pleased with the benefits of living in one.'

Underhill House sits beside a restored 300-year-old Cotswold stone barn near Barton-on-the-Heath, in protected countryside on the Gloucestershire and Warwickshire borders.

It had once been home to a shepherd and was reportedly owned at one stage by Roger Taylor, the drummer with Duran Duran.

Mr and Mrs Seymour-Smith told Grand Designs how they had to come up with a radical solution to appease reluctant Cotswold planning officials - restore the barn and hide the family home underground beside it.

At the time, the plan was described as digging a hole larger than an Olympic swimming pool next to the barn, then putting the family home inside it.

Mrs Seymour-Smith later called the concept 'loft-style living underground'.

The finished house was built from highly insulated concrete and wrapped in heavy insulation and lined with huge walls of triple glazing.

Grand Designs noted that the heat recovery system helped keep the temperature at around 20C, even in winter.

In 2010, the Seymour-Smiths reportedly achieved Britain's best-ever airtightness score. The combined leakiness of the entire house was compared to a hole the size of a squash ball.

When snow fell, the house reportedly stayed warm inside - while snow on the outside of the windows did not melt for four days.

Mrs Seymour-Smith said they did not want 'a hairshirt eco-house, with composting loos', but instead something 'crisp, modern, white and very un-Cotswolds'.

The build was followed by Kevin McCloud for Grand Designs, but unlike many of the show's most memorable projects, it did not descend into disaster.

Mrs Seymour-Smith later joked: 'I didn't get pregnant and we didn't run out of money.'

The couple sold Underhill after deciding to take on another project, refurbishing a Grade II-listed house in the same village.

The hidden eco-home boasts six bedrooms, five bathrooms, five reception rooms, a cinema room, and sits in half an acre of grounds.

The listing on Rightmove states: 'Underhill House is one of the most architecturally and environmentally important private homes ever constructed in the English countryside.

'Conceived, designed and built by architects Helen and Chris Seymour Smith between 2008 and 2010, it achieved international recognition as England's first certified Passivhaus, setting a new benchmark for sustainable rural architecture.

'The house occupies an exceptional position beneath and beside a restored 300-year-old Cotswold stone barn within an Area of Outstanding Natural Beauty near Barton-on-the-Heath.'

A wood-burning stove can be used to heat water occasionally during winter, but when the sun shines 1,000yd of rubber piping in the roof enables water to absorb the heat of the sun and reach temperatures as high as 100C.

The listing continues: 'Temperature and air quality are regulated by an MVHR system (Mechanical Ventilation with Heat Recovery), which extracts warm air from kitchens and bathrooms, filters it and reuses the heat to warm incoming fresh air.

'Hot water is primarily provided by a solar thermal system installed on the south-west-facing roof of the barn, with additional backup from the wood-burning stove or the electric immersion heater.'