Worst case voltage drop.

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Raymond, if the situation is as you describe it, i.e.
no other load on the circuit (a fact which you should
carefully verify by turning the breaker off and
checking what looses power) then there is a problem.
A 15A. branch circuit should be able to supply 1800W
(120V x 15A) with no more than 3% voltage drop or
120V - (.03 x 120) = 116.4V. Even considering an
additional 2% allowable drop for feeders (for a total
of 5%) the voltage at the farthest receptacle under
full load should still be 120 - (.05x120) = 114V.

Of couse, your line volage may be low, so check that.
If so that is an issue for the power company. The 3%
voltage drop is not a hard code requirement but rather
a recommended value in the Fine Print Note (FPN 4) to
section 210-19 of the '96 NEC.

Unfortunately, it sounds to me as though your builder
and his electrican are trying to take advantage of the
average person's inorance about electricity to avoid
making the necessary repair. But double check your facts,
particularly about what else may be on the circuit,
before confronting them again, but then, don't take
no for an answer. Good luck.