The answer can be yes, no, and maybe.
Lengthening or shortening a sailboat is not recommended. There are simply too many variables: center of lateral resistance, sail center of effort, lead and other factors are altered when the length is changed. Designers go to considerable effort to get the relationship of these various factors correct. If they are changed, the results can be questionable.
Most human or motor powered boats can be lengthened or shortened as much as 10%. However, there are exceptions that will be discussed later. Why 10%, can't I go 18%? Possibly, the lines can be lengthened or shortened BUT, we know from experience that up to the 10% factor, the integrity of the design remains basically unchanged. All length factors are altered 10%. This means the location of a centrally located motor, CG (center of gravity, and location of major weights are also shifted by the 10% factor. Beyond the 10% factor the characteristics can be altered; put in another way we get into a gray area. It may be OK but it might not. Then too, the construction may not be adequate when a craft is excessively lengthened. Why take a chance?
Altering length when done properly works well and is simple to accomplish; do it incorrectly and one big mess will probably result. You can't simply add a frame at midpoint or at the stern. A boat has curving lines and each section is (usually) different. Lengthening by this method will cause unfair lines that will be almost impossible to correct and may well alter the hull characteristics. The correct way is to re-space each of the stations or frames from the aft end of the stem to the transom a proportionate amount up to 10%. If frame spacing is equal or varies, multiply by the percentage factor being used. As an example if frames are uniformly spaced 18" apart and the 10% factor is used, each spacing will be increased 1.8" (.10 x 18") or rounded off 1 3/4" (1.75").
Some designs are impractical to lengthen. Stitch and glue boats with full size patterns are a prime example. Simply extending the side and bottom templates for the most part simply won't work and the results can be questionable. Check the web site, many that have tried it had real problems.
Boats that have a sawn harpin are another type that doesn't lend itself to easily lengthening. A harpin is a portion of the boat sheer line as seen in plan view that is sawn to shape rather than bent. If the boat is lengthened in the harpin area the contour of that member changes; the harpin pattern given won't fit properly into the frames. Yes, the harpin shape could be re-drawn, stretching (or shortening) the spacing a proportionate amount. But unless you have some lofting or drafting knowledge, re-drawing the contour may not be easy.
Can the beam be changed? Not practically, this changes all the frame patterns and the curvature (on most) of the side frames. Never attempt to alter the chine beam; this will alter the characteristics to such an extent the result will be unknown.
Can the depth be increased? Possibly a small amount by extending the side frame members equally. This will also increase the beam and could exceed the practical or legal towing width. Boats with excessive flare or tumblehome are best not changed as increasing the side frame member curvature could be a problem.
Every design is different so a universal statement as to whether a design length can be altered is impractical. If in doubt ask the designer.