Home improvement for the Minotaur
Perl 5, 349 bytes

say$p="| "x$_,"+","-"x(16*$n-4*$_+13),"+ $p"for 0..4*($n=pop)+3;say$p="| "x(3*$n+2),"+ | ","| "x2x$n,"+ $p|";$p=~s/..//,$q="-"x(4*$_-1),say"$p+$q+ ","| "x(2*$n-2*$_+1),"+$q+ $p|"for 1..$n;$p=~s/..//;say$p,("+---"."-"x4x$n)x2,"+ $p|";$p=~s/..//,$q="-"x(4*$n+3)."-"x4x$_,say"$p+$q+ | ","| "x2x($n-abs$_),"+$q+ $p|"for-$n..$n;say" "x(8*$n+6),"+----$q+"


(Pass n as a command line argument.)

Computes the maze line-by-line in six sections:


first 4n + 4 lines,
next line (the only line with no -),
next n lines,
next line (the line at the middle of the initial pattern),
next 2n + 1 lines,
final line (the line with leading spaces).