First, I'm no expert, so take what I say with reservation, and check with someone better educated in this field.
Mill Creek is a pretty wide boat, lengthening should make it faster and easier to paddle.
Loading 3 people will mean the boat should sink deeper (slow it back down,) and with less freeboard -- may need to highten the side panel --better check the loading capacity. However, since you lengthen the boat, the designer's original spec. may not apply (may be someone here can help you do the calcilation.)
My 2 cents.
Good Luck.

: I completed a CH LT17 this spring and am having great fun with it. Of course,
: I am now looking to build another boat. I would like a boat that I could
: do some lesurely lake paddling with my wife. I think the boat I am leaning
: towards is the CLC Mill Creek 16.5. My question is if anyone has tried to
: lengthen this boat by a foot or two. The reason is that I would like to
: accomodate one of my small kids in the cockpit and also, it would fit more
: gear when making that trip out to the campsite. I plan to build from plans
: as I also plan to strip the deck. On the plus side, it could accomodate
: more stuff/people and it may be a little faster. On the down side it would
: be heavier and would be difficult to paddle alone. Any insight would be
: appreciated.

: Ray