At one time, the only restrictions placed on a vehicle's height had to do with the size of its tires and total height (a.k.a., bridge clearance). The National Highway Transportation Safety Administration found some time ago that such mandates were inadequate to ensure that the altered vehicle maintained its factory-rated crash rating, or that other vehicles would be able to see them at all. Several states have laws regarding headlight height, and many use such laws in conjunction with those involving bumper and tail light height as well.
Headlight laws serve a dual purpose. The first is that they ensure that an altered vehicle can be seen from the rear view mirrors of a leading vehicle, which can be a problem for frame-dragging custom cars. Secondly, headlight location serves as a reference point for bumper location, by regulating that headlights can only be above or below a certain point.
Headlight Height Requirements
Headlight heights are always measured from ground level to the centre of the lowest bulb on a vehicle's headlight assembly. The follow states mandate that headlight height be between 24 and 54 inches from ground level: Alaska, Arkansas, Washington DC, Indiana, Kansas, Louisiana, Maine, New Mexico, New York, Oklahoma, Oregon and Texas. Colorado mandates a maximum light height (44 inches), but not a minimum.
Headlight height has more to do with bumper height than anything else, which has everything to do with crash protection. Bumpers that are too high will hit the weak glass or B-pillar (the centre roof support) on another vehicle should those vehicles collide in a side impact. The inverse holds true for excessively lowered vehicles, which become a target for everyone else's bumpers in the event of a side impact.
Fog-Lights and Headlights
Headlight heights are measured from the location of the lowest of a vehicle's stock headlight location, not from grille or bumper-mounted fog-lights (which are considered "auxiliary" by most states). What constitutes a "fog-light" vs. a "headlight" can get a little hazy when discussing vehicles like the Subaru STi or Mitsubishi Evolution, which have bumper-mounted fog lights the same size or larger than the actual headlights. In general, it's safe to assume that such a vehicle's "headlights" are exactly where you'd assume they are. Of course, you'd have a hard time lifting a Mitsubishi Evo's front bumper 4 feet off the ground, but the point still stands.
No state explicitly forbids mounting additional lights on a vehicle's roof, hood or roll-bar for use off road. Some local jurisdictions regulate how high the lights may be mounted, but most are fine as long as they're under 13 feet 6 inches. In general, you can mount as many lights as you please wherever you please provided that you never activate them on public roads. The only exception to this rule are swivel-mounted spotlights, which may or may not be legal in your county or parish.