dissenting:
I agree with the view taken by the trial court. Under the circumstances shown, the only fair, reasonable and practical application of the warranty of the trees for a period of one year was to afford opportunity for observation during a full year, including a dormant and a growing season, so it could be determined whether the trees were “sound, healthy, [and] vigorous . . ” as warranted.
The main opinion correctly recites the facts: the defendant planted the trees it sold the city in the fall of 1975. In the spring of 1976, it was found that 12 of the trees were not alive and the city requested the defendant to replace them.
The fact of utmost significance in this case is that it was the defendant who made the request that the performance of his obligation under the warranty be extended, and that he not replace the trees until November 1976, because that is a better time to transplant trees to assure their survival. The city regarded this as a reasonable and practical request and so agreed. That supplemental agreement, extending defendant’s time for planting, was carried out, and the 12 trees were planted in November 1976. But in the following spring, 1977, it was observed that those 12 trees were not alive and therefore did not meet the warranty.
There are two sound and well-recognized principles of law that combine to support the findings and judgment of the trial court. The first is that even though parties may be bound by the terms of a contract, they are free to make collateral or supplemental agreements which they deem to be for their mutual advantage; and this is so even though it may modify the terms of an existing contract.1 And therefore, the agreement to extend the time for planting could reasonably and justly be regarded as extending the warranty for one year from that time.
The other principle is that a party who enters into a contract is obliged to cooperate in good faith to carry out its purpose in accordance with the intention of the parties when it was made;2 and if a party does not do so, he cannot take advantage of his own failure in order to escape liability thereon.3 Applying that principle here, the defendant should not be permitted to take any advantage from his own failure to replace the trees when requested, and thus escape responsibility for meeting the.warranty of the trees as he had agreed.
It is my opinion that the trial court was amply justified in its view that defendant’s request to delay planting the trees extended the warranty; and in so applying the rules of law to the facts as to require the defendant to make good on his agreement *808to plant sound and healthy trees. I would therefore affirm the judgment.
HALL, J., concurs with the views expressed in the dissenting opinion of CROCKETT, C. J.. PLC Landscape Construction v. Piccadilly Fish & Chips, Inc., 28 Utah 2d 350, 502 P.2d 562 (1972), citing Davis v. Payne and Day, 10 Utah 2d 53, 348 P.2d 337 (1960) and Cheney v. Rucker, 14 Utah 2d 205, 381 P.2d 86 (1963).
. Weber Meadow-View Corp. v. Wilde, Utah, 575 P.2d 1053 (1978).
.Watson v. Aced, 156 Cal.App.2d 87, 319 P.2d 83 (1958); Bartlett & Co. Grain v. Curry, 1 Kan.App.2d 242, 563 P.2d 1096 (1977). See Fischer v. Johnson, Utah, 525 P.2d 45 (1974).