כי תבנה בית חדש. אִם קִיַּמְתָּ מִצְוַת שִׁלּוּחַ הַקֵּן, סוֹפְךָ לִבְנוֹת בַּיִת חָדָשׁ, וּתְקַיֵּם מִצְוַת מַעֲקֶה, שֶׁמִּצְוָה גּוֹרֶרֶת מִצְוָה, וְתַגִּיעַ לְכֶרֶם וְשָׂדֶה וְלִבְגָדִים נָאִים, לְכָךְ נִסְמְכוּ פָּרָשִׁיּוֹת הַלָּלוּ (תנחומא): כי תבנה בית חדש WHEN THOU BUILDEST A NEW HOUSE, [THEN THOU SHALT MAKE A BATTLEMENT FOR THY ROOF] — If thou hast fulfilled the command of שלוח הקן (of letting a mother bird go when the nest is rifled), you will in the end be privileged to build a new house and to fulfill the command of “making a guard-rail”, for one good deed brings another good deed in its train, and you will attain to a vineyard (v. 9), fields (v. 10) and fine garments (vv. 11—12). It is for this reason (to suggest this) that these sections are put in juxtaposition (Midrash Tanchuma, Ki Teitzei 1).
מעקה. גָּדֵר סָבִיב לַגַּג, וְאֻנְקְלוֹס תִּרְגֵּם "תְּיָקָא", כְּגוֹן תִּיק שֶׁמְּשַׁמֵּר מַה שֶּׁבְּתוֹכוֹ: מעקה means, a fence around the roof. Onkelos renders it by תיקא; the fencing is like a casing (תיק) which guards things that are within it.
כי יפל הנפל. רָאוּי זֶה לִפֹּל, וְאַעַ"פִּ כֵן לֹא תִתְגַּלְגֵּל מִיתָתוֹ עַל יָדְךָ, שֶׁמְּגַלְגְּלִין זְכוּת עַל יְדֵי זַכַּאי וְחוֹבָה עַל יְדֵי חַיָּב (ספרי): כי יפל הנפל [THAT THOU BRING NOT BLOOD UPON THY HOUSE] IF ANY MAN FALL FROM THENCE — (The words may be taken to mean: if he that is to fall (הנופל) falls from it). This suggests: this man deserved to fall to his death (on account of some crime he had committed), nevertheless his death should not be occasioned by your agency, for meritorious things are brought about through the agency of good men and bad things only through the agency of evil men (Sifrei Devarim 229:7).