Or HaChaim on Exodus 25:8:3

וְעָשׂוּ לִי מִקְדָּשׁ וְגוֹ׳. צָרִיךְ לָדַעַת לָמָּה קְרָאוֹ מִקְדָּשׁ, וְתֵיכֶף חָזַר בּוֹ וּקְרָאוֹ מִשְׁכָּן, דִּכְתִיב אֵת תַּבְנִית הַמִּשְׁכָּן. ועשו לי מקדש, "and they will make a Sanctuary for Me." Why does the Torah refer to the structure as מקדש, Sanctuary, whereas immediately afterwards and many times subsequently it is described variously as "Tabernacle" or "Tent of Meeting?"

וְנִרְאֶה כִּי אָמְרוֹ וְעָשׂוּ לִי מִקְדָּשׁ הִיא מִצְוַת עֲשֵׂה כּוֹלֶלֶת כָּל הַזְּמַנִּים, בֵּין בַּמִּדְבָּר בֵּין בִּכְנִיסָתָן לָאָרֶץ בְּכָל זְמַן שֶׁיִּהְיוּ יִשְׂרָאֵל שָׁם לְדוֹרוֹת. וּצְרִיכִין הָיוּ יִשְׂרָאֵל לַעֲשׂוֹת כֵּן אֲפִלּוּ בַּגָּלֻיּוֹת, אֶלָּא שֶׁמָּצִינוּ שֶׁאָסַר ה׳ כָּל הַמְּקוֹמוֹת מֵעֵת שֶׁנִּבְנָה בֵּית הַמִּקְדָּשׁ, כְּאָמְרוֹ (דברים יב:ט) ״כִּי לֹא בָאתֶם עַד עָתָּה אֶל הַמְּנוּחָה וְאֶל הַנַּחֲלָה״. וְלָזֶה לֹא אָמַר ״וְעָשׂוּ מִשְׁכָּן״ שֶׁיִּהְיֶה נִשְׁמָע שֶׁעַל אוֹתוֹ זְמַן לְבַד נֶאֶמְרָה מִצְוָה זוֹ. וְאַחַר שֶׁצִּוָּה דֶּרֶךְ כְּלָל, אָמַר פְּרָט הַנַּעֲשֶׂה בַּמִּדְבָּר שֶׁאֵינוֹ מָקוֹם לִבְנוֹת בּוֹ בִּנְיַן אֲבָנִים, שֶׁיַּעֲשֶׂה מִשְׁכָּן כַּסֵּדֶר הָאָמוּר. וְתִמְצָא שֶׁכָּתַב רַמְבָּ״ם בְּפֶרֶק א׳ מֵהִלְכוֹת בֵּית הַבְּחִירָה: מִצְוַת עֲשֵׂה לַעֲשׂוֹת בַּיִת לַה׳ דִּכְתִיב ״וְעָשׂוּ לִי מִקְדָּשׁ״, עַד כָּאן, וְטַעְמוֹ הוּא מִשִּׁנּוּי הַלָּשׁוֹן כְּמוֹ שֶׁכָּתַבְנוּ. עוֹד יִרְצֶה בְּאָמְרוֹ וְעָשׂוּ לִי מִקְדָּשׁ כִּי מֵעֵת שֶׁעוֹשִׂים אוֹתוֹ לִשְׁמוֹ יִתְבָּרַךְ, כְּאָמְרוֹ ״לִי״, הֲגַם שֶׁעֲדַיִן לֹא שָׁרְתָה בּוֹ שְׁכִינָה, הֲרֵי הוּא מִקְדָּשׁ וְדִין קֹדֶשׁ יֵשׁ לוֹ. We may assume that the term Sanctuary covers the instruction to erect a Temple both at the time and during later periods of Jewish history. It is a comprehensive positive commandment basically applicable not only in the desert but whenever the Jewish people reside in the Holy Land. Actually this duty is incumbent upon Israel even while the nation is in exile. The only reason this commandment cannot be fulfilled when the people are exiled is that G'd has specifically forbidden that any site other than the Temple Mount in Jerusalem serve as such a Sanctuary once the first Temple had been built by Solomon. Compare the legislation in Deut. 12,9, where G'd speaks of Israel's having attained מנוחה ונחלה, "rest and inheritance," as prerequisites for the erecting of a permanent (as opposed to collapsible) Sanctuary for G'd. This is why the Torah does not introduce this subject matter by referring to a משכן, a temporary structure. Had the Torah mentioned this term first we would have thought that the commandment was of a temporary nature. Once the Torah had introduced the concept of a Sanctuary for G'd it could proceed to give us the details that were applicable in the desert at the time the Torah had been given. You will find that Maimonides writes in the first chapter of the section of his code dealing with the laws applicable to the Temple that it is a positive commandment to build a house for G'd as the Torah has written: "and they shall construct a Sanctuary for Me." He based himself on the fact that the Torah called the Sanctuary משכן already in the very verse following this commandment. Another reason it is called Sanctuary is that as soon as the people would begin construction it was already considered sacred although the structure had not yet been completed and G'd's presence had not yet taken up residence therein.