Say you're cruising the waters of Lake Joseph in your vintage mahogany launch one limpid summer evening, and you pass by this elegant structure perched on the water's edge. You could be forgiven for thinking you're looking at a particularly well-preserved piece of Muskoka history, since, in a sense, you're right - even though, in actual fact, it's just a couple of years old and as well-built as any modern structure on the lake. Its extraordinary authenticity is the result of collaboration between homeowners, designer and builder who, all longtime Muskoka lovers, took pains to get the details right.

The homeowners, Graham and Jennifer Saunders, bought the property in the early 2000s, and while the main cottage was comfortable, it was not distinctive architecturally. "When we bought the property, there was already a double boat slip on the water, but we always had the idea of building a classic Muskoka boathouse, and we wanted it to reflect what we love about Muskoka - the colours, being surrounded by nature, the whole idea of laid-back, relaxed cottage life."

The couple approached a local firm with expertise in the style, Draftech Design in Gravenhurst, with clear ideas about how their "dream" boathouse should look. With the design more or less in place, they approached Taylor Moore of Sherwood North, another longtime Muskoka cottager, to help bring it to life.

One of the interesting things about working on this project, recalls Mr. Moore, is that the region has enacted increasingly strict bylaws governing waterfront structures such as boathouses. The intent, admirably enough, is to exercise some kind of control over both the scenic view for other cottagers and boaters who share the waterways, and to safeguard Muskoka's increasingly vulnerable environment.

In this case, a bylaw enacted in the late 1990s set the maximum

living space in new waterfront structures, such as boathouses, to 650 square feet; the bylaws have since become even more restrictive. For both Draftech and for Sherwood North, says Mr. Moore, "the challenge became how we could configure it to maximize every inch of interior space."

From the outside, the boathouse is the apotheosis of lavish Old Muskoka grandeur, with its twin boat slips, deep roofs and cutout dormers with gable ties, wide upper verandah over a lower covered dock, porthole windows and, of course, quintessential dark siding/white trim. A cupola at the ridgeline adds the final grace note.

The boathouse proper at water level is designed for two boats, and is copiously glazed with both double-hung and decorative windows, such as the jaunty portholes flanking the slip doors. Though designed for the appearance of authenticity, the building is still a modern structure, and state-of-the-art materials keep maintenance to a minimum - always an issue for the boathouse's turn-of-the-20th century antecedents. Prefinished Cape Cod-style cedar siding provides extreme durability with minimal upkeep; in the gables, cedar shakes provide a transition to the cedar-shingle roof - both can last, with minimal care, for another century.

A flight of stairs from the landward path takes you up to the wraparound upper deck, with its traditional picket-and-post railing broken by crossbar panels with glass insets. Furnished, naturally, with a full complement of white-painted Muskoka chairs, the main deck is set before one of two identical entrances, both consisting of big double French doors lit by sidelights and high, arched transoms, letting plenty of summer light into the space.

The living area is arranged under a huge, deeply pitched roof, interrupted by a trio of big dormers at the waterside and another on the landward side. The dormers, along with the oversize gables at each end, maximize usable space inside, though as Mr. Moore points out, since the upper level is essentially all roof, you can't cheat and add storage under knee walls - essentially, there aren't any.

As a result, the interior is as compact and efficient as a sailing yacht. It has a small but well-appointed kitchenette, with white distressed cabinetry on one side and on the other, dark-stained cabinets designed to resemble antiques. Beyond that is a more compact version of a new cottage classic, the great room - only here, the cathedral ceiling is a relatively modest 14 feet at its peak, and the double fireplace, which serves both the main room and the bedroom just behind, features a chimney breast of low-key grey granite veneer instead of granite stone. (Mr. Moore observes that while pink granite may be more emblematic of Muskoka, it's a bit hard to decorate around, especially in such a small space.)

Though the interior of the boathouse is simply designed, every surface was carefully considered and attractively finished. The ceilings are lined with white tongue-and-groove pine; the master bedroom features board-and-batten siding, another rustic touchstone; and the floors are of reclaimed hemlock, a highly durable wood with an attractive grain, which also makes reference to the abundant hemlocks that grow in the region. Even the interior of the main boat area is finished with clear pine boards. (Vintage 20th-century boathouses, of course, were more spartan as a rule, with studs and siding backs left exposed.)

Ms. Saunders confesses that, though the original plan was to make the boathouse's living space a self-contained guest house and continue to use the main cottage for the family, it has had an unexpected pull on the couple and their young family. "The deck, relaxing by the fireplace, or entertaining friends - we've already had some great parties out there. And there's nothing like the feeling of sleeping out here at night, right over the water.

"We've tended to spend more and more of our time in the boathouse - we've fallen in love with it."