First, Hello and welcome to the forum.
awdunstan wrote:If I try Modify -> Lengthen it says "The chosen entity is in a block. Please edit the block". I assume that if I do that it'll change the length of all my wall blocks, not just this one.
Indeed, correct if the original
created block is edited the change will reflect dynamically in all the blocks. A block is a 'grouped' locked collection of entities, each entity within a block can have its own properties - color, line weight, line type etc. and its own layer!
However when a block is inserted in a drawing and say copied several times, any copy of the 'original' block can be modified quite simply by first exploding it and then using the modify tools.
Select the block in the current
drawing (do not edit the original block) and go to Modify > Explode, this will break the selected block back down again into individual entities where you can select them and edit them to change the properties/attributes.
Have a go and see if this takes you further on.
In QCAD 3 there is a wonderful tool called the property editor, this can be used as well as using the modify tools to change an entities properties also.
To use the stretch tool
1.Drag and pull a window around the end of your wall, this is the 'First and Second' corners here.
2. Pick a reference point - any will do.
3. Pick or enter in the command line (as you did before) the relevant coordinate (@ e.g. 100,0 - this will stretch your wall 100 units in the X direction and 0 in the Y direction).