I’m an appraiser and I’ve seen lots of properties with additions over the years. From a valuation standpoint, there is a point of diminishing returns. Unless many of the homes around yours’ have such additions it’s very rare for an addition to add enough value to completely cover the costs. There are a few exceptions, but in those exceptions it tends to be the site that makes it worthwhile – the primary example being oceanfront neighborhoods.
The other thing to consider is the stress that’s involved with such projects. Over the years I’ve seen a lot of divorces brought on by remodel, addition and spec home projects. Many people can’t handle having their lives turned upside down for 6 months or more, and these projects seldom get finished on time or within budget.
Not only that, but these projects seldom turn out as well as originally envisioned.
If you want the bigger/better house I’d recommend going and getting what you want. That way you know exactly what you’re going to end up with. Lots easier and (usually) lots cheaper.