It would be easy to drive straight through the 20th century housing estate and Victorian residential streets of Longley, near Huddersfield, and completely miss the historic gem standing in their midst.

Yet Longley Old Hall, which is barely visible from the main road, is a Grade II Listed 14th century house of national importance. Within its thick stone walls are layers of history which its present owners, Christine and Robin Gallagher, have painstakingly exposed as part of a 10-year renovation plan which developed an unexpected - and life-changing twist.

When the Gallaghers bought the Old Hall in 1998 they assumed it was a 16th century building, but as they began to peel away the 70s décor to expose layer after layer of building developments, they soon realised there was much more to their new home than met the eye.

They called in the conservationists and within a matter of weeks Longley Old Hall was being turned upside-down as its secrets began to unfold. "We had planned to spend at least 10 years restoring the hall one room at a time, at our own pace and to our own budget," said Christine. "But every time we found something new, one simple job led to five others which had to be done before we could start to put everything back together again. Instead of having one room in a mess, almost every room was turned upside-down from the word go."

In less than half their original time-plan, Christine and Robin have spent in excess of £100,000 on renovations alone and anticipate spending at least another £50,000 before the project is complete. Yet the discovery that their home is 200 years older than they first thought, and the interest it has generated among historians and conservationists, has prompted the Gallaghers to open their house for group bookings on certain days of the year, when they give a highly detailed insight into the architecture which spans the centuries under a single roof.

Longley Old Hall was originally owned by the Wood family, whose financial returns of 1379 placed them as the richest family in the area of Almondbury. Through marriage, the Old Hall passed into the hands of the Ramsden family in the mid-16th century, remaining in the family until 1920 when they sold their 4,300 acre estate to Huddersfield Corporation. The only properties excluded from the sale were the Old Hall and an adjoining cottage. These were finally sold in 1977, severing the Ramsden's long-standing connections between the family and the town. But it was only when the Gallaghers bought the property and embarked on their restoration projects that the architectural history started to unravel. "Over the centuries many things had been covered up," said Christine. "It all came to light when I stripped away three layers of pine and plasterboarding in a bathroom and discovered Georgian boarding underneath.

In a gap between the boards were the remains of what looked like a Medieval timber-frame. It was at that point that we thought we'd better take professional advice."

The Gallaghers called in Huddersfield planning department who, in turn, contacted English Heritage and the West Yorkshire Archaeological Service at Wakefield, who confirmed that the panelling was Georgian and should be preserved, and that the Medieval timber was part of an oak brace which needed analysing before the Gallaghers could finish restoring the bathroom. "It gradually dawned on us that this wasn't going to be a straight forward restoration project," said Robin, who is assistant managing director of an industrial property company. "The bathroom was just the beginning. Every time we started stripping walls or removing old plasterwork we found something new which ground the work to a halt again while we waited for the experts to come and take a look."

Their research revealed several major alterations to the house over the centuries.

Documentary evidence suggests that there has been a house on the site since the 14th century and one timber has been dendro-dated to the 1380s, but this is no longer in its original position.

The front elevation of the Hall was built mainly in early or mid-17th century stonework and comprises a central section with cross-wing ranges at either end - creating an H-plan. These wings include mullioned windows on both floors and false gables, added during a major restoration project in 1884.

The central section of the Hall bears plenty of evidence of its complex development over 400 years. The surviving timber-frame replaced an earlier structure, indicated by an elegant ogee doorhead in the east wing. In the late 16th century the whole building benefited from a stone front on the same alignment as the cross-wings. This involved cutting back an eastern tie-beam to accommodate a staircase in the hall of the house. The central truss was incorporated in a wall, supporting the complicated arrangement of roof timbers, and another wall was built to contain back-to-back fireplaces on both the ground and first floors.

A close-studded timber-framed wall on the east wing, believed to date back to the 15th century and indicating the great wealth of the occupants, still survives to this day and is now a striking feature of the hallway. The timber work of the west wing probably dates from the mid to late-15th century, suggesting an extension to an earlier building and indicating the Wood family's rise in status from yeomen to minor gentry.

Alterations in the early 19th century revealed two panels with a painted quotation from St Peter's first epistle, hidden in the kitchen. They were fixed to the drawing room wall until, last year, Robin was granted Listed Building Consent to reposition them.

Almost everything the Gallaghers have touched in the fabric of the building has had to be monitored and investigated, and their original restoration plans had to be modified as a result. Walls they would have originally re-plastered have had to remain in a semi-demolished state of exposed beams and stonework because they are of such historical importance; plans to extend a small 'snug' by a metre ended abruptly when they discovered the remains of a Medieval stone doorway which couldn't be touched; Robin's attempts to install a downstairs loo also ground to a halt when he unearthed the rare 1380 'ogee' lintel and Christine's plans to furnish the house with reproduction furniture and tapestries soon went by the wayside when she realised they were a poor substitute for the real McCoy.

"They would have ruined it," said Christine. "We decided 'less was more' and bought a few very simple early pieces which complement the ages of the house, rather than fill it with inappropriate Medieval-style things. It's been a steep learning curve. I came to this house knowing nothing about history or archaeology. Now my knowledge and interest is so focused I've become more fanatical than the Listed Building people!"

Christine and Robin have now become so adept at pointing out the idiosyncrasies of their extraordinary home that they have opened the house for private viewing to groups of between 10 and 25 visitors, at a cost of £10 a head, and have bank holiday open days for £6 a head. It is a labour of love and barely covers the £1,400-a-year insurance required to open their house to the public, but they are determined to share the glorious secrets of Longley Old Hall.

"At the end of the day this is our home and, as a result, we are very protective of it, but we are equally aware that it is significant part of our heritage," said Robin. "We never expected it turn out like this, but the restoration process has been a wonderful voyage of discovery. We don't regret taking it on for a second. We are just doing what we can to get things right."

For information about Longley Old Hall contact the Gallaghers on 01484 430852 or visit www.longleyoldhall.co.uk

Heather Dixon.