[1 ] Defendant contends the issues as submitted were not broad enough to embrace all the relevant evidence and questions involved. It further contends it was improper to submit the first and second issues because those issues were not controverted in the pleadings, were unnecessary and were confusing to the jury. Defendant submitted three alternative issues that essentially required the jury to decide whether defendant’s operation was in violation of the zoning ordinance. This was not a determination for the jury. The determination of whether a specific use of a piece of property conforms to the zoning ordinance is a question of law. Moyer v. Board of Zoning Appeals, 233 A. 2d 311 (Me 1967); Crary Home v. Defrees, 16 Pa. Commw. Ct. 181, 329 A. 2d 874 (1974). As such, the determination is made by the local zoning board and is reviewable by the courts as a matter of law. The issue defendant contends was for the jury was a matter of law for the courts to determine. Defendant did not seek a ruling from the trial court nor did it raise the issue on appeal. The only factual question for the jury was whether a determination was made pursuant to the zoning ordinance in 1970 that the operation of defendant would be permitted. This was the essence of the third issue presented to the jury for determination.
*413The first and second issues as submitted to the jury did not need resolution. Defendant conceded that a zoning officer made a determination on 2 August 1976 that its ready-mix concrete batching plant was in violation of the zoning ordinance and that it was continuing to operate the facility. Defendant admitted this in the pleadings. It only denied that the use was unlawful. The legality of the determination was not, as we have noted, a question of fact for the jury. Defendant contends it was prejudiced by the submission of these matters on which there was no controversy raised by the pleadings. The trial judge, however, gave the j ury what amounted to a peremptory instruction on the two issues. The only instruction on the two issues was the following.
I will discuss these issues one at a time and explain the law which you consider as you deliberate upon your verdict. The first two are going to be very short and very easy to discuss. The burden is on the Plaintiff on the first two issues. The Court instructs you that if you believe what all the evidence tends to show, you will answer Issue No. 1 “Yes” and Issue No. 2 “Yes.” If you do not believe any of the evidence, you will of course answer them “No.” But if you believe what all the evidence tends to show, you will answer those two “Yes.”
Although it would have been better not to have submitted the two issues to the jury, that error, standing alone, would not require a new trial.
 The third issue submitted to the jury was, “Did the zoning officer of the City of Winston-Salem pursuant to two applications made by the defendant in 1970, determine that the transit mix use of the defendant was most nearly similar to a use or uses designated in the Zoning Table of Permitted Uses under Industrial-2 uses?” In his instruction to the jury, the trial judge placed the burden of proof on defendant to prove this issue by the greater weight of the evidence. In his general instruction on burden of proof, the trial judge stated, “If you are not so persuaded, or if you are unable to determine where the truth lies, it would be your duty to answer the issue against the party with the burden of proof.” He instructed in part on the third issue, *414“If you do not so find or if you are unable to determine where the truth is, then you will answer it ‘No’ in favor of the plaintiff.” Defendant contends the burden of proof of this issue should have been on plaintiff. We disagree. The trial judge properly placed the burden of proof on the third issue on defendant.
Plaintiff alleged that defendant was using the property for the operation of a ready-mix concrete batching plant in violation of the zoning ordinance. Defendant, in its answer, made a denial that its operations violated the zoning ordinance by its specific answers to the paragraphs of plaintiff’s complaint. This was followed by a separate defense to the effect that a city zoning officer had made a determination in August 1970 which approved the use of the property as a concrete mixing plant. Defendant pled this defense “in bar of plaintiffs claim for relief.” This was an affirmative defense to plaintiffs claim that the operation was in violation of the zoning ordinance. “The defendant, of course, has the burden of establishing all affirmative defenses, whether they relate to the whole case or only to certain issues in the case. As to such defenses, he is the actor and has the laboring oar.” Williams v. Insurance Co., 212 N.C. 516, 517-18, S.E. 728, 729 (1937); Jones v. Insurance Co., 254 N.C. 407, 119 S.E. 2d 215 (1961). The city had the burden of proving the existence of an operation in violation of its zoning ordinance. It was defendant’s burden to prove the city had already made a determination that the operation was permissible and did not violate the zoning ordinance.
 Defendant contends the trial judge erred in his instruction concerning estoppel. Estoppel had been raised as a defense to the action. This issue was completely removed from the case as a defense in the first appeal. A city cannot be estopped to enforce a zoning ordinance against a violator due to the conduct of a zoning official in encouraging or permitting the violation. Helms v. Charlotte, 255 N.C. 647, 122 S.E. 817 (1961); Raleigh v. Fisher, 232 N.C. 629, 61 S.E. 2d 897 (1950). The Court in the first appeal of the case recognized that estoppel did not bar a city. The Court also noted that in this case, if a zoning officer approved the use in 1970, “the question of estoppel does not arise because the zoning officer, acting under authority of the ordinance, made a reasonable, justifiable and lawful deter*415mination as to the classification of the use.” 37 N.C. App. at 190, 245 S.E. 2d at 538. The trial judge instructed the jury on the issue of estoppel in a zoning case to the following effect.
There was evidence then that the Hoots Construction Company, which was the correct and proper corporate name in 1970 when the applications were filed and permits were issued, changed its name but remained the same corporation, changed its name to Hoots Concrete Company, Incorporated. That following this there were some complaints to the City Board and that an investigation followed, as a result of which the zoning officer for the City in August of 1976 made a determination, which you will recall, in writing to Hoots Concrete Company and determined that the defendant’s use of the property in question for the operation of a ready-mix concrete batching plant was a violation of the zoning ordinance and did not comply with the zoning ordinance and did not comply with the zoning classification of 1-2 (Limited Industrial). That this determination was served on Mr. Hoots and Mr. Hoots took no action with respect to it, and that thereafter he continued to and did operate a transit mix batching plant at the Indiana Avenue address that we are considering. That thereafter in 1976 the City instituted this action to determine whether or not they were entitled to enforce the provisions of the zoning ordinance.
Now there is, in addition to this, considerable evidence which may have some bearing on your decision that would indicate that the City with its eyes wide open, buying concrete from Mr. Hoots, accepting revenue from him under his license tax, and with the full knowledge of a number of these inspectors, permitted him to continue to operate out there. This may have some bearing on your decision as to whether or not the zoning officer in fact made a decision, but let me caution you that you should not use it for any other purpose. It is not proper that the city officials for a period of time failed or even neglected or even intentionally permitted Hoots Concrete Company to violate the law. To *416 put it as our Court does, “The City may not be estopped.” Now an estoppel is nothing more than you take one position and then you flip-flop and take another. So the fact that he was permitted to do this does not in any way prevent them from coming in here now. (Emphasis added).
Defendant contends this portion of the charge is erroneous and prejudicial for three reasons. (1) Defendant contends the issues had been concluded by the decision of this Court in the first appeal. (2) It was an instruction upon an abstract principle of law not presented by the evidence, pleadings or issues. (3) The close proximity of the estoppel instruction to the erroneous statement that “in 1976 the City instituted this action to determine whether or not they were entitled to enforce the provision of the zoning ordinance” was unduly prejudicial to defendant. Defendant’s first two objections are without merit. The trial judge correctly instructed that estoppel was not a defense. It was a necessary part of the charge. The jurors applying their own sense of equity and fairness might have decided plaintiff should have been estopped where there was evidence to the effect that plaintiff issued business licenses to defendant, purchased concrete from defendant and allowed the operation for a number of years. An instruction on estoppel was necessary in order that the jury would know that the city could not be estopped for those reasons. In the context in which they were given, however, the instructions could have been misleading. The jury could have been led to believe that even if the zoning officer had approved the concrete plant usage, the city could “flip-flop” and if it did so, be allowed “to enforce the provisions of the zoning ordinance” pursuant to the new position it elected to take. The jury could have been confused and determined they had to answer the third issue in the negative because they could not do otherwise or at least such was the opinion of the trial judge. Plaintiffs suit was not to determine whether they were entitled to enforce the zoning ordinance but to enforce the zoning ordinance by an injunction. The erroneous statement about the purpose of the case being a determination of whether the city was entitled to enforce the ordinance followed by a statement that the city could not be estopped from enforcing the ordinance by the action of its officers was prejudicial to defendant.
*417  The trial judge refused to admit in evidence answers relating to uses listed in the table of uses as permissible in 1-2 districts. Defendant’s concrete mixing operation was not specifically described in any of the three types of districts zoned industrial. It was not for the jury to classify the operation. The issue before it was whether a classification had been made in 1970. Plaintiff contends the admission of the evidence would invite the jury to consider an issue of law not before it. We think the excluded evidence should have been admitted in part at least as far as it concerns the procedure for making a determination when the use applied for is not specifically listed in the ordinance. This is relevant evidence of how the zoning officer would have acted when application was made by defendant in 1970. The evidence of similar uses should also have been admitted. The relative similarity of quarry operations in the same zone was relevant to the consideration of whether the zoning official with the power to so do made a determination in 1970 contrary to the determination made in 1976.
The trial judge also excluded testimony that the machinery installed on the site in question cost $94,650.00 and that the total investment of defendant in the site was around $200,000.00. The testimony should have been admitted. It is relevant evidence that defendant had a determination in 1970 of zoning compliance because the jury might have considered that defendant would not have made the expenditure without it. It is not relevant to show detrimental reliance as an element of estoppel because estoppel is not a bar to the city’s enforcement, and the jury should be so instructed.
Defendant’s argument that its motion for judgment notwithstanding the verdict should have been granted is without merit. The evidence was in conflict and should have gone to the jury for its decision on whether a zoning compliance determination was made in 1970.
For the error in the charge where the estoppel instruction followed an inaccurate statement about the issues in the suit and for the errors in evidentiary admissions, defendant is entitled to a new trial. The jury must decide whether a zoning officer approved defendant’s concrete mixing operation in 1970 as a *418permitted 1-2 use. It is defendant’s burden to prove such a determination was made.
Judges Parker and Hedrick concur.