Tree houses are generally thought of as rickety wooden hangouts for kids or the Swiss Family Robinson. But increasingly, adults are muscling in on the act, seeking not only quiet and solitude in nature's bosom, but creature comforts, too. The construction of wooden aeries is branching into new and bespoke areas: a number of European companies are offering solidly constructed, customized treetop retreats which, for prices from $15,000, may be designed to fit almost any species of tree.

While all arboreal dwellings are a treat for the senses — there's the smell of freshly cut timber, the creak of branches and the sound of wind whistling through the leaves — few are as spectacularly luxurious as the upmarket structures created by the Scotland-based TreeHouse Company (www.treehouse- company.com), which look more like mansions than playhouses. The designers can install anything from kitchens and bathrooms to under-floor heating and electricity. The circular cedarwood dining lodge the company erected in an ash in West Sussex, England, for instance, has all that plus a telephone connection, a spiral staircase, 13 windows and a peaked roof. No wonder the private, $185,000 retreat outdoes any earthbound first-class dining hall. "Forty years ago, nobody envisioned things like jacuzzis and log stoves up in the trees," says John Harris, the firm's founder. "But today, nothing is impossible."

Indeed, Harris just completed one of the world's largest tree edifices in

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nginx/1.14.0 (Ubuntu) Northumberland's Alnwick Gardens, scene of the first Harry Potter film. With full disabled access, the giant, $8.5 million complex, which is constructed on stilts around 16 mature lime trees, hosts private and corporate events, and includes a 120-seat restaurant, two classrooms and two private dining rooms.

The tree-house designs of Bremen, Germany-based Baumraum architect Andreas Wenning are more modest in scale, but lean toward the avant garde (www.baumraum.de). A triangular construction, for example, suspended on steel ropes more than 8 m above ground between two beeches, is designed to resemble a ship. The one-room, 7-sq-m dwelling, on the grounds of a livery stable near Bremen, serves as the owner's weekend retreat. It boasts a glass-topped lookout, terrace and hatch-door entry, as well as heating and electricity. So if you have the urge to nest, look out for a suitable tree, or plant one right away — in 20 years' time, it may be ready for your own house.

• URSULA SAUTTER
• Luxury treetop retreats are childhood fantasies for grownups