- Peninei Halakha
A Jewish contractor and Jewish workers may continue building residential homes during the Nine Days in order to sell them, because the units are designed as living quarters and not as luxury homes. In addition, this is their livelihood. Furthermore, if they are building houses in the Land of Israel, this fulfills a mitzva. Plastering and painting, however, should be postponed until after the Nine Days, unless this will cause excessive loss.
Le-khatĥila, one who plans to build or install something superfluous in his home must stipulate with the contractor that he will refrain from working during the Nine Days. If, however, he mistakenly neglected to do so, he should ask the contractor to stop working during this period. Then, if the contractor asserts his right to continue working, there is no need to break the contract with him (mb 551:12, Kaf Ha-ĥayim 551:24).
A Jewish painter or plasterer may not work on another Jew’s home during the Nine Days. However, he may work on the home of a non-Jew. If he guaranteed that he would finish painting a Jew’s home, thinking that he could complete the job before the Nine Days, but is unable to do so in the end, he should ask the customer to exempt him from working during this period. He should even offer to pay the customer a small compensation. However, if the Jewish customer refuses to accept this, and the worker is afraid that he will sustain a great loss – like having to pay a large compensation – he may continue working during the Nine Days.
Poskim generally rule that one should not occupy a new apartment, whether it is owned or rented, during the Nine Days. However, if following such a ruling will cause great financial loss, one may occupy the apartment.
 Further examination is needed to determine whether one may plaster or paint houses that are up for sale, or if one may build mansions in order to sell them. It seems that one may be lenient on this issue if he is liable to suffer a loss, since building luxury items is prohibited only if it is done for a specific Jew, as it brings joy. Preparing houses for sale, however, is considered working in order to make a living, and what I wrote in the main text follows this approach.