The town of Saugus (town) brought this action seeking a declaratory judgment prohibiting the defendant, Refuse Energy Systems Company (company), from challenging $20 million of its $24.4 million property tax assessment. The town claims that the company is bound by contracts it entered into with the town prior to the issuance of industrial revenue bonds used for the construction of a *823solid waste disposal facility. In those contracts, the company agreed not to challenge up to $20 million of the assessed value of its property for three years, beginning January 1, 1976. A Superior Court judge determined this provision of the contracts to be illegal. We granted the town’s application for direct appellate review. We affirm.

We summarize the facts. The defendant company is a joint venture between M. DeMatteo Construction Co. (DeMatteo) and Wheelabrator Energy Systems Inc., a subsidiary of Wheelabrator-Frye Inc. DeMatteo had operated an open dump for about seventeen cities and towns from 1960 through 1975 on land in Saugus, pursuant to a municipal license. Due to complaints about pollution and dangerous conditions at the dump, the town ordered it closed in 1967. The Governor then declared an emergency and ordered the dump to remain open. After litigation between DeMatteo and the town, the town issued an interim license for the dump in October, 1971, based on the condition that DeMatteo construct a solid waste incineration facility to replace the dump.

Also in 1971, the Legislature amended existing development statutes to permit industrial revenue bond financing to be used for solid waste disposal facilities. See St. 1971, c. 1017; G. L. c. 40D. The town began to take steps to use these bonds to finance a solid waste incineration facility through the Saugus Industrial Development Financing Authority (authority). Originally this facility was proposed by an entity known as THESCO, jointly formed by DeMatteo and Combustion Engineering Company.

By December, 1972, the town’s selectmen had approved the project, and the authority had recommended the project for approval by the Commonwealth and had voted its bond resolution, approving a $33 million bond financing. When the company replaced THESCO in 1973 and continued the project, the town’s selectmen and the authority voted again to approve the bond financing.

The company began construction of the facility after it had received a building permit and had spent almost $15 *824million on the project by the fall of 1974. Before the authority would give the company financing of $15.5 million through an interim bond issue, it indicated that the company should agree to terms desired by the town. The company and the town signed a contract on February 26, 1975 (original contract), and the interim bonds were issued shortly thereafter. The original contract provided that the company would “not challenge a real estate assessment by the Town of Saugus of all property owned by [the company] including but not limited to land, buildings, and equipment up to and including twenty million dollars ($20,000,000) as the assessed value for a period of three years beginning January 1, 1976.” In addition, the company agreed to withdraw its pending appeals of real estate taxes before the Appellate Tax Board; not to seek to be classified as a manufacturing facility for tax purposes; to charge the town lower rates for incinerator privileges than it would charge any other town; to build several ponds for the town; and to permit free dumping for the town at DeMatteo’s landfill in Saugus. In return, the town agreed to promote issuance of the bonds and to continue to license the facility.

To complete the project, the company needed another $30 million bond financing to refinance the interim bond issue and to provide additional construction funds. However, prior to this bond issue, the Legislature passed St. 1975, c. 500, which substituted an excise tax of one dollar for each ton of processed solid waste for the property taxes payable on buildings and equipment. See G. L. c. 16, § 24A. This statute contained a grandfather clause, stating that the substitute tax would not apply to a facility owner who had “negotiated agreements containing arrangements relating to real estate taxes or assessments with a city or town” prior to the effective date of the act. St. 1975, c. 500, § 5. The authority, desiring the company to fall within the grandfather clause, refused to issue the $30 million worth of bonds until a new agreement was signed by the company. Consequently, on August 7, 1975, the company signed a second contract with the town (supplemental *825contract), and the bonds were issued later in the month. The supplemental contract primarily “clarified and amended” the original contract to provide that, for at least twenty years, the company’s facility “shall be subject to the local real estate tax of the Town of Saugus in accordance with the laws of the Commonwealth of Massachusetts prior to the effective date of Chapter 500 of the Acts of 1975, except that during the three years covered by [the original contract] as to real estate taxes, the terms of said agreement shall apply.”

The company’s property was assessed at $24,401,500 as of January 1, 1976, and was taxed at that value for fiscal years 1977 through 1981. The company filed applications for abatements and appeals of these assessments for fiscal years 1977 through 1980,1 which are pending before the Appellate Tax Board. The town argues that, under the contracts, the company cannot challenge $20 million of this assessment for the years 1977 to 1979. The Appellate Tax Board decided that determinations of the validity and interpretation of those contracts were beyond its jurisdiction.

The town subsequently filed a complaint for a declaratory judgment.2 G. L. c. 231A, § 2. In its complaint, the town asked the court to find that the two agreements “are valid and enforceable against the defendant,” and that they “require that the pending appeals before the Appellate Tax Board be limited to the issue of the lawfulness of that portion of the Town’s assessment of all of [the company’s] property which exceeds Twenty Million Dollars ($20,000,000).”

The judge below decided that the portions of the agreements intended to forestall the company’s challenge to the first $20 million of the assessed value of its property for the three fiscal years, 1977 through 1979, are “illegal and unenforceable.”3 We agree.

*8261. Ability to challenge assessment. The company claims that the contracts are illegal and unenforceable, because they prevent it from challenging $20 million of its assessment, thus potentially subjecting it to disproportionately high taxes.4 The company supports its argument with the Massachusetts Constitution, Part II, c. 1, § 1, art. 4, which gives the Legislature the power “to impose and levy proportional and reasonable assessments, rates, and taxes.” We have interpreted that section as requiring the imposition of equal tax rates and forbidding “‘the imposition upon one taxpayer of a burden relatively greater or relatively less than that imposed upon other taxpayers.’ See Opinion of the Justices, 332 Mass. 769, 777 [1955].” Bettigole v. Assessors of Springfield, 343 Mass. 223, 230 (1961).

The company claims that the contractual provisions precluding it from challenging its assessment violate this general rule. The fair cash valuation standard for real estate assessments “cannot be varied by public officers or by agreement of parties.” Waltham Watch & Clock Co. v. Waltham, 272 Mass. 396, 412 (1930).

When contracts provide for waivers of tax appeals or tax exemptions, they must be entered into with the clear approval of the Legislature. Thus, we will not enforce such contracts without legislative approval. Cf. Collector of Taxes of Boston v. National Shawmut Bank, 259 Mass. 14, *82720 (1927). Because the Legislature has preempted the field of taxation, its approval is required. “The laws established by it cannot be waived or changed by municipalities or their officers. Such laws are and must be general in their operation. When the Legislature has covered the whole subject, there is no room for the exercise of authority by local officers. The town has no power to make a contract concerning that subject.” Southborough v. Boston & Worcester St. Ry., 250 Mass. 234, 239 (1924). “Moreover, what property is taxable and what is exempt is a subject covered by laws which are and must be general in their operation. They cannot be varied by a contract between the town and the taxpayer.” Assessors of Dover v. Dominican Fathers Province of St. Joseph, 334 Mass. 530, 537 (1956).

However, despite clear authority prohibiting such contracts, the town presents several arguments in favor of the validity of the contractual provisions at issue. The town asserts that the Legislature has delegated the power to assess property to municipalities. Therefore, it argues, the town may contract with taxpayers to have them waive their right to challenge part of their assessment. The flaw in the town’s argument is that an enforced waiver may allow municipalities to impose disproportionate tax burdens indirectly, through the taxpayers’ inability to challenge an assessment. In the case before us, the town’s position could result in the company’s paying taxes in excess of those required by law.5 The delegation of the power to assess does not negate general tax policies of the Commonwealth.

In addition, contrary to the town’s assertion, the Legislature has not delegated to municipalities the power to make agreements such as those at issue here. The town states that G. L. c. 58A, § 6, and G. L. c. 59, § 64, allow agreements between assessors and taxpayers. These provisions, however, permit agreements on abatements of taxes only after *828an assessment has been made and an appeal has been filed. Moreover, neither of these statutes provides for an advance waiver of rights to appeal assessments.6

Furthermore, it is not clear that the contractual provisions at issue were designed or would operate as merely a waiver of the company’s right to challenge $20 million of the assessed value of its property. We believe that the contractual arrangement was more than a simple waiver: it was intended to impose a minimum tax on the company. If the company’s property is assessed for more than $20 million, but its value is actually less than that amount, the company would be illegally forced to pay more taxes than other similarly situated taxpayers.7

Although the town argues that the company would not have been forced to pay taxes on a $20 million assessment if the property had been assessed at a lesser value,8 its statements elsewhere belie that view. The town has referred to the arrangement in its brief as a “minimum assessment,” which would provide the town with a “method of stabilizing, for only three years, revenue that the Town was already authorized to levy and collect.” Furthermore, the tow;n had earlier called the provision a “minimum requirement” to provide it with “the equivalent of a million dollars net for 20 years,” or a “guarantee of tax payment on $20,000,000.” These statements demonstrate that the town *829may have enforced the agreement as requiring a minimum tax, rather than as a mere waiver of the right to appeal designed to free the town from the cost of abatement appeals.9

The agreements therefore dictate the imposition of a tax regardless of the true value of the property. The situation is analogous to Southborough v. Boston & Worcester St. Ry., 250 Mass. 234 (1924), in which the defendant agreed to pay the plaintiff $900 annually in exchange for permission to construct a street railway on particular public ways. The court interpreted this payment to be part of the excise tax. “If such excise was less than $900, the company was to be credited with the amount thereof on account of such annual payment, and if the excise exceeded such sum in any year, then such excess was to be paid by the company to the town.” Id. at 236-237. Although the excise tax had been repealed for the years in question, the judge decided the case on the ground that the contract was “invalid and nonenforceable,” id. at 241, because it “relates to the subject of taxation, which in essence is and must be authorized by a general law,” and not by a contract between the taxpayer and the town, id. at 240.

Southborough controls the case before us because, as we have said, the waiver of the right to appeal operates as the imposition of a fixed tax regardless of the actual value of the property.10 We therefore conclude that the provisions in *830the original and supplemental contracts, which prevent the company from challenging $20 million of its property tax assessment, are illegal and unenforceable.

2. Estoppel and loches. The town suggests that the judge erred in deciding that neither estoppel nor loches bars the company’s assertion of the defense of illegality. We believe the record supports the judge’s conclusion.

The company should not be estopped, because it does not appear that the town “has been induced by the conduct of [the company] to do something different from what otherwise would have been done and which has resulted to [its] harm and that the other knew or had reasonable cause to know that such consequence might follow. But the doctrine of estoppel is not applied except when to refuse it would be inequitable.” Boston & Albany R.R. v. Reardon, 226 Mass. 286, 291 (1917). The company’s claim may not be barred by loches either, because there was no delay by the company in asserting its claims which prejudiced the town. Three Sons, Inc. v. Phoenix Ins. Co., 357 Mass. 271, 278 (1970).

In this case, the only act the town took which it might otherwise not have taken was to issue the bonds, and it has alleged no real harm to it resulting from the conduct. Furthermore, there is evidence in the record supporting the company’s claim that it asserted the illegality of the contracts before signing them. The town claims that it was also harmed by having collected taxes from the company, which it has used for its expenses. However, as the judge found, this prejudice to the town is the same prejudice it faces from any application for abatement before the Appellate Tax Board. Thus, we conclude that these defenses are without merit.

3. Conclusion. We do not reach other questions presented concerning the validity of the contracts. The trial judge determined that there was no justiciable controversy be*831tween the parties regarding other contractual provisions, and found that those provisions have been performed.11 Although the company did not appeal that decision, it requests us to invalidate the contracts in their entirety as illegal preconditions to the issuance of bonds under G. L. c. 40D. We need not address that issue. “Although a party may defend a judgment on any ground asserted in the trial court, failure to take a cross appeal precludes a party from obtaining a judgment more favorable to it than the judgment entered below.” Boston Edison Co. v. Boston Redevelopment Auth., 374 Mass. 37, 43 n.5 (1977).

Therefore, we affirm the judgment of the Superior Court that the contractual provisions intended to prohibit the company from challenging up to $20 million of its assessment are invalid and unenforceable.

Judgment affirmed.