(dissenting, with whom O’Connor, J., joins). The court’s decision to reverse the judgment is based on an issue of statutory interpretation that was never raised by the plaintiff (and, hence, not responded to by the defendants) in the administrative proceeding or at trial, and that is not *666discussed by either party in the briefs submitted to us. It is unusual for us to allow an issue to be raised for the first time on appeal. See Mass. R. A. P. 16 (a) (4), as amended, 367 Mass. 921 (1975); Milton v. Civil Serv. Comm’n, 365 Mass. 368, 379 (1974). It is still more uncommon for us to decide a case on the basis of an issue not even addressed in the parties’ briefs; on the infrequent occasions when we have done so, the result has been to affirm a judgment on alternative grounds, not to reverse that judgment. See Commonwealth v. Elder, 389 Mass. 743, 746 & n.6 (1983); Beeler v. Downey, 387 Mass. 609, 613-614 n.4 (1982); Royal Indem. Co. v. Blakely, 372 Mass. 86, 87-88 (1977). Having considered the language and history of G. L. c. 148, § 26A1h, I conclude that the court has incorrectly interpreted the statute. Additionally, I reject each of the issues actually raised by the plaintiff, and would, consequently, affirm the judgment. I therefore dissent.
The pertinent paragraph of G. L. c. 148, § 26A1/2, reads as follows:
“Every building or structure of more than seventy feet in height above the mean grade and constructed prior to January first, nineteen hundred and seventy-five, shall be protected with an adequate system of automatic sprinklers in accordance with the provisions of the state building code; provided, however, that sprinklers shall not be required to be installed in patient rooms in hospitals, or in public or private libraries, or in houses of religious worship; provided, further, that sprinklers shall not be required to be installed in buildings where construction has commenced prior to January first, nineteen hundred and seventy-five and which have been submitted to the provisions of chapter one hundred and eighty-three A; and provided, further, that automatic sprinklers shall not be required in rooms or areas of a telephone central office equipment building when such rooms or areas are protected with an automatic fire alarm system” (emphasis added).
The statute establishes a general requirement, followed by provisos offering specific and limited exceptions, one of which exempts buildings owned under the provisions of the condominium statute, G. L. c. 183A. General Laws c. 148, *667§ 26A1/z, makes no explicit exception for buildings owned by cooperative corporations, which are subject to the requirements of a separate enabling statute, G. L. c. 157B. We must construe an unambiguous statute as it is written. Rambert v. Commonwealth, 389 Mass. 771, 773 (1983). Provisos that create exceptions to the general coverage of a statute should be strictly construed, and where a statute provides a number of express exceptions, no other exceptions should be implied. District Attorney for the Plymouth Dist. v. Selectmen of Middleborough, 395 Mass. 629, 632-633 (1985). See Woods v. Executive Office of Communities & Dev., 411 Mass. 599, 604-605 (1992); 2A C. Sands, Sutherland Statutory Construction §§ 47.08, 47.11 (5th ed. 1992). Such is the case here. The specificity of the reference to the condominium statute, and the lack of reference to the cooperative statute, indicate clearly that the Legislature intended to exempt only condominiums, and not cooperatives, from the requirements of G. L. c. 148, § 26Alh. “The fact that each is governed by separate legislative enactments fortifies the conclusion that the word ‘condominium’ does not include within its meaning the term ‘cooperative.’ ” Bronstein v. Prudential Ins. Co., 390 Mass. 701, 704 (1984) (holding that provision in G. L. c. 121A, § 18D, regulating conversion of rental units in redevelopment projects to condominiums, does not apply to conversion of such units to cooperatives). Moreover, legislation on a matter of public safety should be construed so as to best effectuate its purpose. Pruiss v. Springfield, 21 Mass. App. Ct. 960, 961 (1986), citing Whirlpool Corp. v. Marshall, 445 U.S. 1, 13 (1980). Broadening the condominium exception to include cooperatives will mean that more high-rise buildings will lack sprinkler systems, contrary to the stated puxpose of G. L. c. 148, § 26AV2.1
The court offers a plausible rationale for treating both *668cooperatives and condominiums differently from residential buildings owned by landlords. One may, just as plausibly, (1) cite various dissimilarities between cooperatives and condominiums, (2) argue that cooperative residents are more accurately described as tenants than as owners, and (3) therefore conclude that cooperative structures are more akin to apartment buildings than to condominiums.2 However, opinions as to the most logical categorization of cooperatives are not relevant to interpreting the meaning of the disputed exception in G. L. c. 148, § 26 A1/z.3 Regardless of whether the Legislature should have treated condominiums and cooperatives alike, in fact it did not do so. By allowing cooperatives to benefit from the condominium exemption in G. L. c. 148, § 26A:/z, the court substitutes its policy judgment for that expressed in the statute, in a fashion that is “tantamount to adding to a statute a meaning not intended by the Legislature.” Mitchell v. Mitchell, 312 Mass. 154, 161 (1942). We are not free to add language that the Legislature chose not to supply.4
Having concluded that the plaintiff’s cooperative was not *669entitled to the statutory exemption for condominiums, I would respond as follows to the three issues raised by the plaintiff at trial and on appeal:
(1) The plaintiff sought a declaration that G. L. c. 148, § 26A1h, violates the equal protection and due process clauses of the United States Constitution and the Massachusetts Declaration of Rights by exempting condominiums, but not cooperatives, from the sprinkler installation requirement. Because the statute implicates neither a suspect class nor a fundamental right, its classification will be upheld if it is rationally related to the furtherance of a legitimate State interest. See FCC v. Beach Communications, Inc., 508 U.S. 307, 313-314 (1993); Dickerson v. Attorney Gen., 396 Mass. 740, 743 (1986), and cases cited. The plaintiffs bear the “burden of proving the absence of any conceivable grounds which would support the statute.” Zeller v. Cantu, 395 Mass. 76, 84 (1985), quoting Commonwealth v. Franklin Fruit Co., 388 Mass. 228, 235 (1983). In this case, the judge found that differences in the ownership status of condominium unit owners and cooperative shareholder tenants provide a sufficient rational basis for the statutory distinction. I suggest that a matter of practicality offers an additional basis for the classification. The Legislature may have thought that it would be more difficult to implement the installation of sprinklers in condominiums than in cooperative units. In a condominium building, the managing entity has control of the common areas, but may lack authority to undertake improvements within the individually owned units; by contrast, a cooperative *670corporation, as owner of the entire building, has authority over the interiors of the leased units. Compare G. L. c. 183 A, §§ 4, 10, with G. L. c. 157B, §§ 4, 5. Either of these explanations is more than sufficient to survive the plaintiffs’ constitutional challenge.
(2) The plaintiffs had sought a variance, to allow a system of partial installation of sprinklers. They argued at trial that the board’s denial of this variance was an abuse of discretion and should be reversed, in accordance with G. L. c. 30A, § 14. The judge concluded that the board had acted within the authority delegated to it by G. L. c. 6, § 201, and that the plaintiffs had failed to prove that the board’s decision was arbitrary and capricious.51 agree with the judge’s conclusion.
(3) Anticipating that it might not succeed on its other arguments, the plaintiff sought a declaration that converting the cooperative building to condominium ownership would exempt it from the sprinkler installation requirements of § 26AV2. The plaintiff argued that, under the language of the statute, a building constructed before 1975 that is subsequently converted to a condominium may gain the benefit of the condominium exemption, regardless of when the conversion occurs. The judge concluded that the exemption applied only to those pre-1975 buildings that were already condominiums before the statute became effective.6 This conclusion is correct. The interpretation sought by the plaintiffs would allow owners of pre-1975 buildings to evade the sprinkler requirement by converting their structures to condominiums, thus thwarting the public safety purpose of the statute.
I conclude that the judge properly rejected all the plaintiff’s arguments. I would affirm the judgment of the Superior Court.
The statute, inserted by St. 1986, c. 633 (entitled “An Act requiring the installation of automatic sprinklers in buildings over seventy feet in height”), was enacted in the aftermath of a serious fire at the Prudential Building, which then lacked a sprinkler system. Rep. A.G., Pub. Doc. No. 12, at 67, 68 (1988). The initial version of the bill, 1986 House Doc. No. 862 (introduced a week after the Prudential fire, 1986 J. House 3, 2407), exempted only patient rooms in hospitals. The proviso exempting condominiums was added later by amendment (as were the exceptions for libraries and areas within telephone equipment buildings). See 1986 Senate Doc. No. 2202, § 2, at 5. The statute was later rewritten to include houses *668of worship. St. 1989, c. 306. This history indicates that the bill has been amended in response to specific complaints about its general coverage, and suggests that the lack of an exception for cooperatives was neither unintentional nor inadvertent.
As shareholders in the cooperative corporation, cooperative residents own an interest in the building as a whole, not in their individual units; the cooperative corporation is the landlord, and the residents are tenants occupying apartment units under proprietary leases with the corporation. See Bronstein v. Prudential Ins. Co., 390 Mass. 701, 704 (1984); G. L. c. 157B, § 4 (definition of “proprietary lease”). Condominiums, on the other hand, involve exclusive ownership and possession of individual units, with unit owners having the right in common to use the common elements. See Bronstein, supra, G. L. c. 183A, §§ 1, 3, 4. See also 4B R. Powell, Real Property par. 633.1 [3], at 773-774 (P. Rohan ed. 1992) (distinguishing condominiums and cooperatives); D. Chuman, F.S. Jackson, & E.L. He-bard, Condominiums and Cooperatives 185 (2d ed. 1984).
The logic of the statutory distinction may be relevant to determining whether the classification meets the rational basis test for constitutionality. This issue is discussed below.
Noting that the term “cooperatives” is absent from both §§ 26AV2 and 261 of G. L. c. 148, the court argues that if cooperatives are subject to § 26A1 / 2 because of their absence from the listed exceptions, then they are not subject to § 261 because of their absence from the list of buildings to which that statute applies. The court concludes that this would be an absurd result. The court’s conclusion overlooks the differences in the language by which the coverage of each statute is defined. The language of § 26A1h is exclusive, while that of § 261 is inclusive. In § 26A1h, *669cooperatives are absent from a list of exceptions to a general rule, and as we have discussed above, such exceptions are narrowly construed to restrict their scope. In § 261, cooperatives are absent from a list of specified residential buildings that begins with the phrase “including, but not limited to.” As cooperatives are akin in type to the examples specified in the list, they may be included within the coverage of § 261 even though not specifically mentioned. See Mahoney v. Baldwin, 27 Mass. App. Ct. 778, 779-780 (1989), and cases cited. Hence, while neither statute mentions cooperatives, both statutes include cooperatives among the structures required to have sprinklers. Overall, the various sprinkler statutes treat condominiums and cooperatives alike, except that the Legislature has chosen to grant an exemption to older, high-rise condominium buildings.
In any event, § 261 is a local-option statute that only affects structures in those municipalities that have accepted the law, so that its coverage is of limited relevance to interpreting § 26A1/£, which instead applies throughout the State.
The board found that the plaintiffs plan would not provide life safety equivalent to that of a full sprinkler system, and would put residents and fire fighters at greater risk. The building’s prior history supports this assessment. On January 21, 1974, a fire in the building was confined to the unit where the fire had begun, but the occupant was killed.
We have held that buildings constructed before 1975 and converted to condominiums in 1981, before the effective date of G. L. c. 148, § 26A1/a, see note 1, supra, were entitled to the condominium exemption. Brook House Condominium Trust v. Automatic Sprinkler Appeals Bd., 414 Mass. 303, 306-307 (1993). In that case, we expressly declined to rule as to whether the exemption would be available to buildings converted after the date of the statute. Id.