Section 107.51(b) is the altitude rule remote pilots quote and then misapply. The small unmanned aircraft cannot be higher than 400 feet above ground level, unless two things are both true: it is flown within a 400-foot radius of a structure, and it does not fly higher than 400 feet above the structure's immediate uppermost limit. No nearby structure means no exception. Class G does not create an exception. A LAANC grid does not create an exception above the altitude you were authorized, and it never authorizes more than 400 feet AGL on its own.
The structure exception, with the arithmetic
A 700-foot tower, with the aircraft remaining within 400 feet horizontally of it, allows flight up to 400 feet above the top. That is about 1,100 feet AGL. The wrong answers are "still 400 AGL," which ignores the exception, and "700 AGL," which stops at the top of the tower and forgets the 400 feet above it. The horizontal piece matters. If you drift farther than 400 feet from the structure, you are back to the 400-foot AGL cap, even if you are still below the tower's top.
The same arithmetic works on a shorter structure. A 600-foot building, with the aircraft remaining within 400 feet of it, allows 1,000 feet AGL. Stopping at the roof leaves you at 600. Adding 400 feet twice and answering 1,400 double-counts the exception.
Obstruction heights on a sectional are how you find the top. The bold number is MSL. The parenthetical is AGL. Add 400 feet to the top of the structure, not to a random MEF in the quadrant. Symbols are covered on the sectional symbols page.
Clouds can be lower than 400
The altitude cap is not the only vertical limit. You also stay 500 feet below the clouds and 2,000 feet horizontally from them, with at least 3 statute miles visibility from the control station. If a cloud deck sits at 800 feet AGL, 500 feet below it is 300 feet AGL, and that is your ceiling even though 400 would otherwise be legal. If the deck is at 1,200 feet and there is no structure, cloud clearance alone would allow 700 feet AGL, but 400 feet AGL is more restrictive, so 400 controls. Compare the limits. Do not apply only the one you remember. The visibility and cloud page is the other half.
Waiver versus authorization
Section 107.51, including the 400-foot limit, is on the §107.205 waiver list. A survey that needs 550 feet AGL over open terrain in Class G, with no structure, is not "impossible to approve" and it is not legal with no approval. The operator may apply for a waiver of §107.51 and has to show an equivalent level of safety. LAANC is the wrong form. LAANC authorizes entry into certain controlled airspace up to a grid ceiling. It does not waive an altitude limit in Class G, and it does not authorize flight above 400 feet AGL anywhere.
Flying at 300 feet inside Class B still needs a §107.41 authorization even though you are under 400. The altitude rule and the airspace rule are independent. Being legal on one does not satisfy the other.
The datum is above ground level, not mean sea level. A site at 5,000 feet of elevation is still limited to 400 feet above the local ground, not 400 feet MSL. High density altitude can make that climb harder to achieve. It does not raise the legal cap. Performance is the density altitude page.
Groundspeed has its own number in the same section: 87 knots, which is 100 miles per hour. Altitude questions sometimes offer that speed as a distractor. It is not a height.