Considering there are a couple of extended 53 on the market for less than $200,000 and we got a quote of $150,000 to add an extension to ours, which we did not do, I think it makes a lot more sense to either buy one that is already done or by a 58 yachtfish. The advantage to an added cockpit on a 53 is you still retain the full size of the aft deck. John
Financially it does make more sense to buy one already done or buy a 58 yes.
If one was to do an extension it makes more sense if you're in need of or about to do a complete hull repaint. Then you can amortize the painting cost on the need of a repaint.
From what I know, and I may be wrong, is if your boat is painted with Imron or Alexseal you can fair in the paint on an extension, or any spot repair required, topside or hull, without having to repaint everything. Apparently Awlgrip is not as ideal a paint...unfortunately most Hatteras's seem to have been originally painted with Awlgrip.
We used Imron when we did a complete hull paint at the time of the extension and therefore anytime I needed to do any repairs or mods it was much easier. I did become somewhat of a self taught expert in glass work and HVLP spray techniques and subsequently did my own work on other repairs and painting on the boat on the years to follow. (For the extension we did, we did hire a very knowledgable fibreglass guy who was actually building truck caps at the time - we used the correct fibreglass cloth, resins and fairing pastes for the job - he did such good work I eventually had him build me the fibreglass swim platform you see in the picture to replace the original teak one).
I will add that before we embarked on the extension we did research the process some, talking to some of the yards in South Florida that had done some and also engaged a naval architect (Steve Killing) to provide some advice on the design and issues we'd encounter - a conversation with him was a good investment for advice only...no drawings...he simply said "follow all the lines of the boat back to the length you want and add ballast..you'll be fine".
As for the cost, if you were to find a qualified fibreglass guy who knows how to build or mold fibreglass parts, and a yard that allows you to bring in your own team and do some DIY, you could do the extension itself for probably under $75,000 and do it in an off season complete (5-6 months), (with some good preplanning of course)...if you're willing to do some of the work and research. You unfortunately have to open up parts of your aft stateroom and be prepared for some cleaning up along the way. That cost could include stainless tanks for additional fuel, water and holding tanks which become the additional ballast you need with an extension due to additional bouyancy. Trimming out the boat with the additional bouyancy was not a big issue at all.
Such a project is not for the faint of heart - unless you get one of the yards to do the work and I know there are a few that are very good at it but - it will of course, cost you.