This case was tried before the district court on a stipulation of facts agreed to by the parties. Plaintiff-appellant Gordon Paving operated a rock crusher and an asphalt plant on land in Hidden Hollow, Blaine County. The use antedated the effective date of Blaine County’s first zoning ordinance. Hidden Hollow was zoned as low density residential, but since the use preceded the zoning ordinance it became a valid non-conforming use.
After the effective date of the ordinance Gordon Paving sought to modernize its asphalt plant. Accordingly, it applied to Blaine County’s zoning board (hereinafter referred to as Blaine County) for a variance. Blaine County granted the' variance, but conditioned it on the removal of the entire operation from Hidden Hollow within three construction seasons. It is disputed whether Gordon Paving ever accepted the condition, even though no appeal was made at that time. Gordon Paving modified its original asphalt plant replacing some obsolescent components with a more efficient system. As part of the same modernization plan, Gordon Paving discontinued the use of its rock crusher and moved it to a different location in Blaine County. Gordon Paving operated its plant as modified for three construction seasons. At the end of the 1973 season when the conditional variance was about to expire, Gordon Paving instituted proceedings challenging the validity of the variance. After exhausting its administrative remedies, it appealed to the district court. The district court upheld the validity of the variance and its terms and this appeal followed.
The threshold issue on appeal is whether a variance was necessary under Blaine County’s zoning ordinances to authorize Gordon Paving’s modifications of its Hidden Hollow plant. The other issues raised on appeal — whether a three year termination period was a reasonable condition to attach to a variance and whether Gordon Paving acquiesced in the condition — are moot if Gordon Paving’s modifications could have been implemented without a variance. We agree with Gordon Paving that a variance was not necessary.
In reaching this conclusion we note that judicial review of decisions of municipal zoning authorities is limited. As administrative bodies having expertise in the zoning problems of their particular jurisdictions, their actions are presumptively valid. Where there is a basis for a reasonable difference of opinion, a court may not substitute its judgment for that of the zoning authority. Ready-to-Pour, Inc. v. McCoy, 95 Idaho 510, 511 P.2d 792 (1973); Cole-Collister Fire Protection District v. City of Boise, 93 Idaho 558, 468 P.2d 290 (1970); 3 Anderson, American Law of Zoning § 21.916 (1968).
The deference paid to the expertise of zoning authorities does not allow a zoning board to reach a decision that is not supported by substantial evidence, however. Although the scope of review is narrow, a decision will not be sustained if there is no evidence which supports it or if it is supported by evidence which falls short of being substantial. 3 Anderson, American Law of Zoning § 21.17 (1968).
Applying these principles to the case at bar we reach the conclusion of law that a variance was not necessary to effectuate Gordon Paving’s modifications. Since a record of the administrative proceedings was not maintained, we base our decision upon a review of the stipulation of facts agreed to by the parties. Blaine County’s position that its zoning ordinances require a variance is not supported by substantial evidence. The relevant zoning ordinances require a variance for non-conforming changes of use or for enlargements or extensions of non-conforming uses. The modifications implemented by Gordon Paving do not fall under either of these categories.
On appeal Blaine County relies exclusively on the argument that Gordon Paving’s modifications constituted an enlarger *732ment or extension of an existing use.1 In support of this theory, Blaine County points to the fact that the size of Gordon Paving’s modified asphalt plant was larger than the prior plant. It is undisputed, however, that Gordon Paving was originally operating a rock crusher as well as an asphalt plant as part of one operation at the Hidden Valley site. Gordon Paving moved its rock crushing facilities to a new location when it modernized its asphalt plant. The size of Gordon Paving’s total operation was actually decreased after the modifications. At the most this case involves a reasonable substitution of more modern facilities for obsolescent equipment. Generally, such a substitution does not constitute an enlargement or extension. Chilson v. Board of Zoning Appeals, 344 Mass. 406, 182 N.E.2d 535 (1972); Eitnier v. Kreitz Corporation, 404 Pa. 406, 172 A.2d 320 (1961); E. C. Sehnieder, Inc. v. Zoning Board, 389 Pa. 593, 133 A.2d 536 (1956).
The only other evidence that Blaine County has introduced is that Gordon Paving’s volume of output increased. Blaine County conceded, however, that this increased capacity resulted from the greater efficiency of the modernized equipment. The operating time and the environmental impact of the plant were substantially reduced despite the increase in volume. Case law in other jurisdictions has held that as a matter of law an increase in the volume of use is not an enlargement or extension. 1 Anderson, American Law of Zoning § 6.32 (1968). The same result must certainly inhere when increased volume is accompanied by a greater compatibility with the surrounding locale.
The fact that Gordon Paving initially requested a variance is not proof that a variance was required. Blaine County’s zoning ordinances are determinative. A zoning commission cannot predicate the application of a zoning ordinance on a petitioner’s erroneous assumption that it was applicable.
Judgment reversed. Costs to appellants.
BAKES and BISTLINE, JJ., concur.. Blaine County admitted that Gordon Paving’s modifications did not entail any change in the basic nature of the operation. Both before and after the modifications, Gordon Paving was engaged in asphalt production by the same basic process. As a matter of law, no change of use occurred.