The approach evokes the dream home: a thicket of pines along a winding drive, obscuring a 19th-century mansion tucked along Wissahickon Avenue in West Mount Airy.

The call of cicadas drowns out the sound of gravel crunching under the car's tires. As the car moves around the drive's last bend, the manse reveals itself in all its gothic glory. Ahhh ... home.

It's easy to understand why George Clifford Thomas and his wife, Caroline, rarely left this estate during the late 1800s. Instead, the couple tended their 23 acres with the help of a small staff.

But now, years of tree growth hide a large part of Thomas Mansion. A swath of lawn, a reflecting pool filled with water lilies, and a large conservatory long ago succumbed to wind, rain, and tree roots. Much of the house's grandeur has long since faded.

This week, a new kind of tenant will move in. The Fairmount Park Historic Preservation Trust, a not-for-profit that manages and restores properties throughout the park, will move its headquarters into the house. The relocation represents the trust's third occupation/restoration project.

Namesake Thomas, a prominent investor and businessman, retired at 40. He held 11 mortgages, and at his death in 1907 he left his entire estate to the City of Philadelphia, which eventually absorbed it into Fairmount Park.

Much of the former estate took on parts of the benefactor's name: The house became Thomas Mansion, the surrounding estate became Clifford Park, and the hill on the northwest side of the house was called Tommy's Hill, considered for generations one of the best sledding hills in the city.

Despite Thomas' wish that his estate remain open to the public, by the late 1920s a park administrator had co-opted it for use as his personal fiefdom. Similarly, in the mid-1970s an investigation by the Philadelphia Daily News found city employees living rent-free in several park houses, including Thomas Mansion. By 1987, public outcry stopped yet another park administrator from an extravagant renovation of the house for use as a luxurious office.

"The trust was formed as a means to deal with underutilized, abandoned, and quite possibly falling-down buildings in the park, of which there were many," said Hanley Bodek, president of the trust's board.

The trust began after then-Councilman Michael Nutter introduced legislation in 1993 to create a new management system for the park's historic houses. More than 30 properties were falling into disrepair. Bidding rules had hindered swift action on repairs, and bylaws had prevented the city from entering into long-term leases with tenants who could pay rent and maintain the properties at no cost to the taxpayer.

"The ultimate goal is for us to move out," Bodek said of the new headquarters.

The trust's first office at 16th and Walnut Streets was well outside the park. Its previous two headquarters, Sedgley Estate and Ridgeland Mansion, both found new occupants.

The trust moved onto the Sedgley Estate in the late 1990s and restored the old porters house. In 2001, the keys were handed over to Outward Bound, a nonprofit educational organization.

Staff then moved into the old sheep barn located near Ridgeland Mansion. That property was added to the Wellness Community's growing campus this year. The community, a cancer-support organization, matched a $500,000 grant from the state to renovate more than 6,000 square feet of the barn for reuse for research, meetings, and mind/body programs.

The trust expects to restore Thomas Mansion for $1.2 million raised through grants and donations, considerably less than one outside estimate that came in at $4.5 million.

With park budgets slashed and a large stock of architectural treasures in constant danger of falling into disrepair, trust officials have a growing sense of urgency.

"We want our buildings to be self-sufficient," said Lucy Strackhouse, the trust's executive director. "With the economic conditions, our program is even more important now than in the 1990s."

Built in 1869, Thomas Mansion represents one of the very few High Victorian houses in trust. The peaked gables and Wissahickon schist give the house enough gravitas that it was once short-listed for use as mayoral mansion. But Philadelphians have historically frowned on imperial trappings for their executive.

"We work on so many late- 18th-early-19th-century buildings that we are all so happy to be working on a Victorian with interesting finishes and possibilities," said Jessica Baumert, the trust's senior conservator. "There's just a little more drama."

The house feels almost comical in its eccentricities. A huge, cantilevered porch hood juts out 13 feet from the entrance and protects an intricate marble mosaic floor. Carved roping introduces itself there and later weaves throughout the interior. Affectations of an English hunting lodge nudged their way into the design. Original carved wooden fox heads will be recast in plaster and returned to their place of honor at the peak and base of the hood.

Inside the 17-room mansion, the faint smell of mildew mixes with latex paint and fresh plaster. There, the carved roping joins coffered ceilings and wainscoting. Alternating two-inch strips of light and dark wood flow throughout the main floor, providing a modern twist.

A second kitchen, added in 1889, has a cathedral ceiling pierced with triangular windows finished in etched glass.

Another cathedral ceiling graces the library, with original shelving. The marble fireplace supports a plain cross, one of many found throughout the house that hint at Thomas' religiosity. An inglenook fireplace outside the "new" kitchen bears the inscription "Give us this day/ Our daily bread/ Want not waste not."

No one knows for sure who the architect was, though conservators suspect it was either Frank Furness or Samuel Sloan. The trust has announced a contest awarding $100 to anyone who figures it out.

Sara Rogers, an intern from the University of Pennsylvania's Historic Preservation Program, spent three months researching the house at 15 area libraries and archives. She compiled a thorough history, but still couldn't definitively identify the architect.

"It's such an interesting, significant house, you'd think there'd be more on it," she said by phone.

Rogers conducted paint analysis with her classmates that revealed beautiful sage greens and brick reds on exterior windows, which were formerly covered in what Strackhouse dubbed "asylum gray."

"I learned a lot about theory, conservation, and lab work at school," Rogers said. "But this is a really good opportunity to apply those things and get the practical experience."

"If the city was in charge of this it would be hard to imagine them coordinating interns, too," Bodek said of the educational programs.

As in their previous occupation/restoration projects, trust apprentices have set up a workshop on the Thomas property.

Elsewhere, the trust is in negotiations to lease Glen Fern, the 1747 Colonial once owned by miller/mogul Thomas Livezey.

Outside Strawberry Mansion sits the trust's latest charge: Horace Trumbauer's Strawberry Mansion Music Pavilion. The red-brick ruin is one of a series of pergolas built by the architect. The medley of crumbling arches overgrown with vegetation and open to the elements suggests a Fairmount version of Tintern Abbey.

Thomas Mansion remains structurally sound, although it has sustained substantial water damage. The history of its reclusive owner and his architect has yet to be fully researched. A future tenant needs to be identified.

But for now, outside the kitchen, a single window restored by Rogers and her classmates inspires the trust's senior conservator. "Even if it's just restoring one window, it's nice to see the impact you're making," Baumert said. "It gives a vision to everybody of what it will look like. Hopefully, that will help get the whole building restored."