[Lam 2:8 KJV] (8) The LORD hath purposed to destroy the wall of the daughter of Zion: he hath stretched out a line, he hath not withdrawn his hand from destroying: therefore he made the rampart and the wall to lament; they languished together.

2:8 חָשַׁב יְהוָה לְהַשְׁחִית חֹומַת בַּת־צִיֹּון נָטָה קָו לֹא־הֵשִׁיב יָדֹו מִבַּלֵּעַ וַיַּֽאֲבֶל־חֵל וְחֹומָה יַחְדָּו אֻמְלָֽלוּ׃ ס

2:8 καὶ ἐπέστρεψεν κύριος τοῦ διαφθεῗραι τεῗχος θυγατρὸς Σιων ἐξέτεινεν μέτρον οὐκ ἀπέστρεψεν χεῗρα αὐτοῦ ἀπὸ καταπατήματος καὶ ἐπένθησεν τὸ προτείχισμα καὶ τεῗχος ὁμοθυμαδὸν ἠσθένησεν

In Zechariah it appears to be a negative thing:

[Zech 1:16 KJV] (16) Therefore thus saith the LORD; I am returned to Jerusalem with mercies: my house shall be built in it, saith the LORD of hosts, and a line shall be stretched forth upon Jerusalem.

I always thought from context that "stretching out a line" was a positive thing but that does not appear to be so. Is is possible that it was used to mark off trees, old structures and such for demolition before new building?

It seems to me to match both contexts.

[Lam 2:8 KJV] (8) The LORD hath purposed to destroy the wall of the daughter of Zion: he hath stretched out a line, he hath not withdrawn his hand from destroying: therefore he made the rampart and the wall to lament; they languished together.

2:8 חָשַׁב יְהוָה לְהַשְׁחִית חֹומַת בַּת־צִיֹּון נָטָה קָו לֹא־הֵשִׁיב יָדֹו מִבַּלֵּעַ וַיַּֽאֲבֶל־חֵל וְחֹומָה יַחְדָּו אֻמְלָֽלוּ׃ ס

2:8 καὶ ἐπέστρεψεν κύριος τοῦ διαφθεῗραι τεῗχος θυγατρὸς Σιων ἐξέτεινεν μέτρον οὐκ ἀπέστρεψεν χεῗρα αὐτοῦ ἀπὸ καταπατήματος καὶ ἐπένθησεν τὸ προτείχισμα καὶ τεῗχος ὁμοθυμαδὸν ἠσθένησεν

In Lamentations it appears to be a negative thing.

However, in Zechariah it appears to be a positive thing:

[Zech 1:16 KJV] (16) Therefore thus saith the LORD; I am returned to Jerusalem with mercies: my house shall be built in it, saith the LORD of hosts, and a line shall be stretched forth upon Jerusalem.

I always thought from context that "stretching out a line" was a positive thing but that does not appear to be so. Is is possible that it was used to mark off trees, old structures and such for demolition before new building?

It seems to me to match both contexts.