How to achieve a straight line when not flush to required wall?
I'd set a couple of 2x4 stakes in the patio area as an offset line, parallel to the brick but out of the way of the new wall. This will allow you to set your final lines appropriately. 

Establish your first parallel line with a level


Drive a stake in at an arbitrary location with the width perpendicular to the garage foundation down at the level of the wall. 
Take a round number measurement to the center of the stake top from the brick. Set another stake roughly at that location.
Now take a more precise measurement to a mark at the approximate center of the stake using a level. Stand the level on the stake with one edge at the center and set the bubble plumb. Now measure between the level (when plumb) and the brick. 
Transfer that dimension to the other stake using the level. Keep the level plumb and move it until you have the correct dimension, then mark the stake. You now have marks creating a line parallel to the brick. 




 |________________________________________________| &lt;-- garage foundation

 **************************************************
 *                                                *
 *                                                * &lt;-- patio area outline
 *     _                                  _       *
 *    |.| &lt;--- stakes with nails ------&gt; |.|      *
 *    |_|      equidistant from brick    |_|      *
 *                                                *


Create a perpendicular line using the 3:4:5 technique


Set a small nail in each stake and pull a dryline between them.
Measure between the nails and find a slighly smaller dimension that's a round multiple of 3. For example, if you measure 123 inches, mark 120. This is the second corner of your 3:4:5 triangle. The first was the nail from which you measured. 
Pull a tape measure from point 1 (the nail) extending roughly perpendicular to the dryline. Find a dimension that's 4/3 of the earlier measurement. (Multiply that number by 4 and divide by 3. For 120 you'd get 180.)
Strike an arc on the ground or some scrap lumber at that distance, a foot or so both ways from your estimate of perpendicular. 
Have an assistant hold the "dumb" end of your tape measure on the mark you made on the dryline earlier without moving the dryline. Find where a number that's 5/3 of your original dimension intersects with the arc from the previous step. That point lies on your second reference line, which will be perpendicular to the first line.
Set a stake and a nail at that point and pull a line back to the original nail.




 **************************************************
 *                                                *
 *     point 1 (nail)           point 2 (mark)    *
 *     _                          |       _       *
 *    |.|  ---- multiple of 3 ----*----  |.|      *
 *    |_|                                |_|      *
 *     |                                          *
 *     |                                          *
 *     | &lt;-- measure a multiple of 4              *
 *     |     from point 1                         *
 *     |                                          *
 *     |                                          *
 *     |                                          *
 *     |                                          *
 *     |                                          *
 *     |                                          *
 *     |                                          *
 *     |                                          *
 *     |                                          *
 *     |                                          *
 *     |                                          *
 *    \_/ &lt;-- strike an arc                       *
 *                                                *


From there you can take measurements to parallel for your actual wall positions. The drylines can stay in place during wall construction for reference.