Witness A. E. Gentry, an experienced building contractor, testifying for plaintiffs, gave as his opinion that the value of the building on the date possession was delivered to plaintiffs was from $7,500.00 to $8,000.00. Over objection of defendant he testified it would have been worth from $9,000.00 to $9,500.00 had it been constructed according to contract. On cross-examination he stated: “I don’t know a thing about the Endsley house, I never have seen it.” Defendant moved to strike Gentry’s testimony concerning value. The court overruled the motion to strike. This was error.
The contract provides that the building “shall be exactly like house built on Endsley Ave. house # 13” (with .minor exceptions) and shall be constructed of “the same kind of material used in Endsley Ave. house #13.” These -are the plans and specifications. Plaintiffs offered no evidence whatsoever as to the plan of or materials used in the *666Endsley Avenue house. The witness Gentry never saw it and was not qualified to testify what the value of the building would have been if constructed “exactly like” and of the “same kind of material” as the Endsley Avenue house. A witness is not competent to testify to a fact beyond his personal knowledge or to base an opinion upon facts of which he has no knowledge. Rankin v. Helms, 244 N.C. 532, 540, 94 S.E. 2d 651; Warren v. Insurance Co., 215 N.C. 402, 404-5, 2 S.E. 2d 17; Harrison v. Railroad, 194 N.C. 656, 660, 140 S.E. 598.
Defendant insists that its motion for nonsuit should have been allowed since there was no competent evidence on the part of plaintiffs as -to the extent, if any, of their damages and no evidence upon which the jury could have based an award of damages. Plaintiffs’ evidence makes out a prima jade case of breach of contract with respect to the quality of workmanship. “Where plaintiff proves breach of contract he is entitled at least to nominal damages.” Sineath v. Katzis, 218 N.C. 740, 756, 12 S.E. 2d 671. See also Tillis v. Cotton Mills, ante, 359. The court correctly overruled the motion to nonsuit.
“The fundamental principle which underlies the decisions regarding the measure of damages for defects or omissions in the performance of a 'building or construction contract is that a party is entitled to have what he contracts for or its equivalent. What the equivalent is depends upon the circumstances of the case. In a majority of jurisdictions, where the defects are such that they may be remedied without the destruction of any substantial part of the benefit which the owner’s property has received by reason of the 'contractor’s work, the equivalent to which the owner is entitled is the cost of making the work conform to the contract. But where, in order to conform the work to the contract requirements, a substantial part of what has been done must be undone, and the contractor has acted in good faith, or the own-erbas taken possession, the latter is not permitted to recover the cost of making the change, but may recover the difference in value.” 9 Am. Jur., Building and Construction Contracts, sec. 152, p. 89; Twitty v. McGuire, 7 N.C. 501, 504. The difference referred to is the difference between the value of the house contracted for and the value of the house built — the values to be determined as of the date of tender or delivery of possession to the owner.
Since there must be a new trial the following observations are in order. Defendant’s evidence tends to show that such defects as do exist may be readily remedied without substantial destruction of any part of the building. Should the jury accept this view, the measure of damages is the cost of labor and material to make the building conform to the contract. Moss v. Knitting Mills, 190 N.C. 644, 649, 130 *667S.E. 635. Plaintiffs’ evidence tends to show that in order to remedy deficiencies a substantial part of what has been done must be undone. If the jury accepts plaintiffs’ theory of the case, the measure of damages is the “difference in value” rule stated above.
Higgins, J., took no part in the consideration or decision of this case.