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Restoration of old houses is always full of surprises – things are seldom quite what they appear to be. "I assessed it as a good solid 18th-century wall," recalls Michael West, associate director of engineer FaberMaunsell, referring to just one of many mirages he's encountered on his current structural engineering project at the Grove, near Watford. "I then discovered that the previous occupant had taken the wall down, created a cavity and filled it with insulation just because the room behind was cold."

The Grove is a mansion that is being converted into a five-star hotel by independent hotel group Ralph Trustees. West explains that the offending wall had been underpinned to take the weight of an extra storey and was solid at foundation level. "We only discovered the cavity when someone took off a coping stone and said 'there's a hole there'." The contractor, Try Construction, was just about to extend the wall up into the roof using brick, but this discovery meant a last-minute change to a lighter, timber solution.

This situation is just one of the many challenges of this refurbishment. West has found the Grove has suffered more than its fair share of problems dating back over 150 years. On top of this, the project is especially demanding because of the change of use, which means it needs much higher levels of servicing, sound insulation and structural integrity than a private home or a visitor attraction. English Heritage is also keeping a close eye on the refurbishment as the house is grade II* listed.

The Grove will have 227 bedrooms in both traditional and contemporary styles, complete with state-of-the-art technology, when it is finished next summer. A golf course has been created in 300 acres of rolling parkland and an ornamental lake has been restored. A spa is being built and a health and leisure club. Two hundred of the bedrooms will be situated in a massive extension to the house, which will also boast a ballroom and wedding reception suite. But the restored original house will be the pièce de résistance, as it will have the priciest bedrooms and the best restaurants.

The aim is to restore the house to how it was in the days of the Earls of Clarendon who owned it from 1740 to 1923. It was built in three phases, with architect Matthew Brettingham constructing the first part, rebuilt from a much older manor, in 1740. In 1780 Sir Robert Taylor was appointed to extend the house. In 1842 the Clarendons decided they needed more room for servants to look after the guests at their lavish parties so they appointed Edward Blore, famous for his work on Buckingham Palace, to extend still further by adding a third storey.

In 1923 the Clarendons finally packed up and left. The Grove is thought to have been used for educational purposes until 1939 when the London Midland and Southern railway took it over as its headquarters in a move named "Project X" – LMS had been based in central London but the imminent threat of war prompted its relocation. After the war, in 1947, British Rail took on the house as a training centre until privatisation in 1994, when it was put up for sale. When Ralph Trustees took possession of the house in 1996, the Grove was very neglected and had been hacked about to insert numerous small offices in the grand rooms.

The Grove's colourful history had taken its toll and when FaberMaunsell took on the job, the team found the house was suffering from serious structural problems. For instance, when Blore added the third storey 160 years ago, he ran the new floor joists longitudinally over the existing partition walls, which did not all extend to the ground. "Amazingly this didn't break the floors," says West, "but it subjected them to severe distress." It was imperative this problem be fixed because of all the extra weight the structure had to take: the luxury bathrooms are fitted out with marble and glass, the bedrooms are laden with heavy fabrics and the floors have added sound-deadening layers.

The solution was a combination of new beams and strengthened existing beams. Fortunately, the external walls were sound and so could be used to help remedy Blore's mistake. "We cut through the partitions and used new steel beams to transfer the loads to the outside walls at second-floor level," explains West. At first-floor level there were existing beams, formed from squared-off tree trunks called bressummers, supporting internal floor loads and partitions. To strengthen these so as to take the additional loads, a large groove was cut into the beams and a T-section steel plate glued into position.

Another major structural problem centred on a large brick flue extending up the house, installed by Blore. British Rail had cut an opening in the middle of it, using two railways sleepers as a lintel and supporting the flue on two brick piers. "The survey seemed to show this wall sat on a brick arched vault below, but I thought it couldn't have done – it's daft," says West. "I brought the surveyors back to redo it, but they were right." The vault had to be partially demolished, a foundation built for the flue, and the vault rebuilt using lime mortar. A steel beam was inserted higher up to prevent the flue buckling.

The structural engineer found that even fixing small problems could take months. For example, West was concerned about a crack in a brick wall on a staircase. "I had to get down on bended knee and beg English Heritage to allow me to take the plaster off. I couldn't for the life of me understand why the wall had cracked." Three months later, West was given the go-ahead. He found a deep channel had been cut into the wall to accommodate the wires and pulley for the Victorian bell system for calling servants.

Getting five-star services, such as mechanical cooling, into the house has also proved challenging. The plant has been installed in the new extension and pipes run in a service tunnel to the old house. Luckily, the main hallway wall features a recessed niche originally used for hand washing, created by installing a false wall a small distance from the original. This proved a handy void for running pipes up the house. From here, pipes and ducts are run in redundant chimneys and through voids in the floors and ceilings.

A new main entrance and reception area has been grafted onto the plain north facade. This extension is in the style of the original house, but it had to be designed to keep the additional weight away from the old foundations of the mansion. This was done by supporting the extension three metres away from the mansion and cantilevering the structure across to the old house. It features cavity wall construction, but the outer leaf of brickwork is set in lime mortar to match the mortar joints of the main house. Because this outer skin is so weak, West has used a cast concrete inner leaf to support the building.

It has taken three years to get to this stage, yet the house still looks little more than a shell. Drylining is now being installed in the mansion. Soon the serious business of installing all the fittings can begin in earnest. And when visitors are relaxing in their Jacuzzis next year, they will have no idea about what lies behind the plasterboard.