I'd suggest you get a "pre-survey" then correct eveyrhting that brings out and then go for documented formal survey. THat will maximuze valuation and document (in word writing) exactly what you have and real $$ value. Interview the surveyor before you hire him so you know his attitude and approach and choose the one that will work with you and meet the needs of your survey.
It's kind of like estate vs tax vs market appraisals in real estate. Each type of appraisal/survey is different depending on goal(s).