From guest cottage to house...; MODULE BY MODULE

Since last September, when they moved out of their former home, Phil and Jackie Tuttle and one son, who returns from school during the holidays, have lived in a tiny guest house - an 18-by-20-foot structure that includes a kitchen , living room, bathroom, storage room, and two sleeping lofts.

Accustomed as they were to a much larger house, they were surprised at how ''livable'' the tiny cottage is while they await the construction of a new home overlooking the water at Gun Point near here.

A couple on their own could live in moderate comfort in such a cottage, or even a single parent with a child, says Mrs. Tuttle.

The Tuttles are not alone in their view. Small but increasing numbers of people, particularly young couples starting out, contend that ''going small'' is a lot better than no house at all, particularly if the alternative is a rented studio apartment.

''If you can live in a small apartment, you can readily adapt to the small home'' is the reasoning of these small-home owners.

In fact, the innovative use of space in the tiny apartments in Manhattan and other large cities has, in part, stimulated the equally creative use of space in the small house. This guest-cottage approach particularly appeals to the new breed of owner-builders. There are even plans available where a single-guest house can be expanded over the years into a home of more conventional size.

In fact, the innovative use of space in the tiny apartments in Manhatten and other large cities has, in part, stimulated the equally creative use of space in the small house. This guest-cottage approach particularly appeals to the new breed of owner-builders. There are even plans available where a single-guest house can be expanded over the years into a home of more conventional size.

One such set of plans is put out by the Cornerstones Energy Group, an owner-builder school here in Brunswick. It is the initial stage of this home that is currently housing the Tuttles.

What starts out as a 12-by-20-foot cottage ends up as a 22-by-40-foot house that includes a deck and greenhouse/sunroom.

The idea behind the plan is to ''start small'' and then add the three remaining units, one by one, as the owners can afford it. The design is so straightforward that anyone with basic carpentry skills could build it.

Charles Wing, who designed the basic unit over lunch with some students several years ago, says it can be looked on as a ''four-year house'' by someone starting out. By that he means you could put up one new unit each year until the full-sized house is achieved ''and you wouldn't have to look too hard to find the time to do it in.''

This way, too, you might also get by without having to seek out a bank loan with its attendant high interest rates.

Mr. Wing, a former scientist with the National Aeronautics and Space Administration, and students from Cornerstones erected the basic module, complete with kitchen cabinets, plumbing, and electric wiring at a Brunswick energy fair in exactly three days. It doesn't always go that smoothly, he admits , and so he suggests that do-it-yourselfers allow 11/2 man-hours per square foot of living space.

Qualified carpenters could do the same job in an hour or less per square foot with ease, he says.

Qualified carpeneters could do the same job in an hour or less per square foot with ease, he says.

Current material cost in the Northeast for the basic module (kitchen/dinette, living room, and sleeping loft) comes to around $4,500, excluding the refrigerator and kitchen range. I suspect most folks would want to include the bathroom and storage/bedroom addition right away, which would boost that figure somewhat.

Like all energy-efficient construction these days, the design has large windows facing south to let in the warming winter sun as well as tight, well-insulated walls and roof to slow down heat loss once the sun has set.

Several small basement-style windows provide ventilation.

The sleeping lofts are, in effect, large bunk beds that provide a comfortable sitting room but no standing room. There is a reason for this: The height has been limited to the maximum that can be trucked by law. In other words, the design is such that it can be built in the driveway of a city home and then trucked to another location, say alongside the lake at a country campsite.

Indeed, the Tuttle cabin was built in Brunswick and hauled as is to Gun Point some 15 miles away. When their house is built, the Tuttles will use the cottage to house guests.

In the past decade house prices have tended to increase at around 10 percent a year. So even a small cottage can provide a meaningful hedge against property inflation.

Plans for the passive solar guest house include a perspective rendering, floor plan, elevations, roof and floor framing, window and kitchen-bath detail, and building section.

For more information, write Cornerstones, Cumberland Street, Brunswick, Maine 04011.