State of Minnesota, Respondent, vs. Leo M. Thibodeau, Appellant.

may not be cited except as provided by

Minn. Stat. § 480 A. 08, subd. 3 (1996).

 STATE OF MINNESOTA

 IN COURT OF APPEALS

 C3-96-2419

State of Minnesota,

Respondent,

vs.

Leo M. Thibodeau,

Appellant.

 Filed September 30, 1997

 Affirmed in part and reversed in part.

 Crippen, Judge

Hennepin County District Court

File No. 96059396

Hubert H. Humphrey, III, Attorney General, Suite 1400 NCL Tower, 445 Minnesota Street, St. Paul, MN 55101; and

Craig M. Mertz, Craig M. Mertz Law Office, Suite 210, 600 West 79th Street, P.O. Box 652, Chanhassen, MN 55317 (for Respondent)

John F. Bonner III, Malkerson Gilliland Martin LLP, Suite 1500 AT&T Tower, 801 Marquette Avenue, Minneapolis, MN 55402-3205 (for Appellant)

Considered and decided by Parker, Presiding Judge, Crippen, Judge, and Short, Judge.

 U N P U B L I S H E D O P I N I O N

 CRIPPEN, Judge

Following a bench trial, appellant Leo Thibodeau, who acted pro se, was convicted of three zoning misdemeanors: (a) expanding a nonconforming use by constructing a deck; (b) expanding a nonconforming use by adding a shed; and (c) constructing the deck without a building permit. Appellant contends that the deck and shed did not "extend or intensify" his mobile home as proscribed by ordinance and that the building permit prosecution was flawed because the city did not prosecute the offense until four years after the act occurred. We affirm the two counts of expanding a nonconforming use but reverse the conviction of failing to obtain a permit.

 FACTS

In 1991, appellant moved into a 50 year-old mobile home park in the City of Mound. Sometime in the 1960's, the City of Mound annexed a portion of the park. The city's zoning ordinances did not permit mobile home parks, but this park was allowed to continue as a nonconforming use. Appellant's storage shed was on his lot when he moved into the park.[1]

In the spring of 1992, appellant constructed his deck. A few months after its construction, he filed an "after the fact" building permit application with the city. In April 1995, the city's building inspector notified appellant that he would have to obtain variance approval from the city council before a building permit could be issued. The inspector stated that the mobile home park was a "nonconforming" land use and construction of a deck was a prohibited expansion. Appellant did not apply for variance approval.

In July 1996, appellant was charged with one count of violating the City of Mound's zoning ordinance, section 350:420, subd. 7, for expanding a nonconforming use by building a deck. Later, the city amended its complaint to include two additional charges: (1) expanding a nonconforming use by installing a storage shed, in violation of section 350:420, subd.7, and (2) failing to obtain a building permit for the construction of the deck, in violation of section 300:15, subd. 9.[2] Appellant was found guilty on all counts.

 D E C I S I O N

A reviewing court need not give deference to a trial court's decision on a purely legal issue. Frost-Benco Elec. Ass'n v. Minnesota Pub. Utils. Comm'n, 358 N.W.2d 639, 642 (Minn. 1984). Interpretation of an existing zoning ordinance is a question of law. Frank's Nursery Sales, Inc. v. City of Roseville, 295 N.W.2d 604, 608 (Minn. 1980). The court must (1) construe the zoning ordinance according to its plain and ordinary meaning; (2) construe the zoning ordinance strictly against the municipality and in favor of the property owner; and (3) consider the zoning ordinance in light of the underlying policy objectives. Id. at 608-09.

Deck as extension

Appellant argues that his deck is not an unlawful extension to his mobile home because the zoning ordinances did not define the term "extend" or specifically prohibit the construction of a deck. Normal maintenance of a building or other structure related to a nonconforming use, such as a mobile home, is permitted if it does not extend or intensify the nonconforming use. See Mound, Minn., Code of Ordinances § 350:420, subd. 7 (1993). But the plain and ordinary meaning of "extend" includes a deck as a prohibited alteration. See County of Freeborn v. Claussen, 295 Minn. 96, 100, 203 N.W.2d 323, 326 (1972) (holding addition to existing building is clearly extension or expansion of prior nonconforming use); The American Heritage Dictionary of the English Language 647 (3rd ed. 1992) (defining "extend" as "to enlarge the area, scope or range of").

Moreover, in identifying the existing mobile home park as a nonconforming use, it is apparent the city's policy objective is not only to regulate the park but also to provide for its gradual elimination. See County of Lake v. Courtney, 451 N.W.2d 338, 341 (Minn. App. 1990) (holding that underlying policy of nonconformity ordinances is to "regulate existing non-conformities and to provide for their gradual elimination"), review denied (Minn. Apr. 13, 1990). The trial court did not err in concluding that appellant's deck was an unlawful extension to his mobile home.

Appellant also asserts that neither the deck nor the shed are expansions because they do not expand the size of the mobile home park. Appellant cites no authority that limits the term "extension" or "expansion" to mean that it must increase the size of the park.

2. Permit violation

Appellant asserts that he should be exempt from prosecution because the city waited over four years to charge him with constructing the deck without a permit.

Appellant was convicted of a misdemeanor for failing to obtain a building permit before constructing his deck. The criminal statute of limitations for misdemeanors, as for most other offenses, is three years, unless the crime fits within one of enumerated provisions. Minn. Stat. § 628.26(i) (1996).

Respondent concedes that "the criminal violation occurred when the construction occurred" and makes no effort to account for the lapse in time between the offense and the charge. Appellant constructed his deck in the spring of 1992. The city charged him with failing to obtain a building permit on November 22, 1996, four and a half years later. There is nothing in the record to explain why the city did not prosecute the charge when it occurred. We reverse appellant's permit conviction, but this does not affect the court's sentence.

3. Other issues

Several additional issues warrant discussion, including appellant's contentions that the city's zoning ordinances do not apply because the mobile home park existed prior to the city's annexation; that his shed is a permissible accessory building; and that the city is not entitled to enforce its zoning ordinance because his deck and shed existed for a number of years. We find all these arguments are without merit.

a. SLS Partnership v. City of Apple Valley

Appellant contends that the city's zoning ordinances do not apply because the mobile home park existed before the city annexed the area.

Generally, a zoning regulation may not operate to retroactively deprive a property owner of a previously vested right or of a use his property enjoyed before the regulation came into effect. Fairmount Township Bd. of Sup'rs by Fairmount Township Zoning Bd. v. Beardmore, 431 N.W.2d 292, 294-95. (N.D. 1988). Residential zoning ordinances may constitutionally prohibit the creation of uses that are nonconforming but existing nonconformities must either be permitted to remain or be eliminated by use of eminent domain. Hooper v. City of St. Paul, 353 N.W.2d 138, 140 (Minn. 1984).

Appellant argues that, following the court's reasoning in SLS Partnership v. City of Apple Valley, 511 N.W.2d 738 (Minn. 1994), the zoning ordinances of Minnetrista Township should govern the mobile home parcel. Appellant erroneously interprets SLS Partnership.

In SLS, the court held that upholding Apple Valley's demand that a mobile home park comply with new set back and lot size requirements would allow "the City to achieve by zoning ordinance that which can only be done by eminent domain." Id. at 742. Application of Apple Valley's restrictive ordinances, in light of new requirements on designs and sizes of mobile homes, would have forced the park to remove 50% of the mobile homes already within the park. In contrast, in the case before us, the city does not seek to enforce new zoning restrictions but lawfully restricts any extension or intensification of a nonconforming use. See Freeborn County, 295 Minn. at 99, 203 N.W.2d at 325 (permitting municipality to disallow expansion or enlargement of preexisting nonconforming uses).

b. Shed

Appellant contends that his shed is not a nonconforming use because it is a permissible accessory building. Appellant argues that because the mobile home park is zoned R-1, single family residential, his shed is allowable under statute as an accessory to a single-family residence.

An accessory use is a structure subordinate to the principal use structure on the same lot. Mound, Minn., Code of Ordinances § 350:310, subd. 3 (1993). A 'principal use' may be either permitted or conditional. Mound, Minn., Code of Ordinances § 350:310, subd. 144 (1993). The mobile home park is neither a permitted nor a conditional use; an R-1 residential district has one dwelling per parcel and the mobile home park has twelve dwellings per parcel. And there is no evidence that appellant's separate mobile home lot conforms to the lot requirements of an R-1 residential lot. Appellant's shed is not an accessory building under the zoning ordinances and is prohibited.

c. Enforcement not time-barred

Appellant contends that Mound is prohibited from prosecuting him for his expanded use violations because the deck and the shed have existed for several years and the city had knowledge of their existence. But prohibited zoning ordinance uses are ongoing. Where placement of a structure is prohibited, the violation continues so long as the structure remains in place. See SLS Partnership 511 N.W.2d at 743 (holding the city is entitled to enforce set back requirements even though requirements had not been previously enforced).

  Affirmed in part and reversed in part.

[1]Appellant acknowledged at trial that the shed was his property.

[2]Appellant argues the complaint amendment never occurred. The record shows that the city amended the complaint in the trial court's chambers before the trial commenced and that appellant was found guilty on all three counts.