QUOTE(weikee @ Jun 25 2017, 02:57 PM)
There are height limit, and also need to have openings. Cant recall the details. You need to go to your location town council to find out. Each town council will have different rules.
The height of the exterior common wall between link houses is not so much of an issue with the local town council. In other words, the town council will likely not bother or care since it's a minor issue. There are more serious cases such as extension of the kitchen either single-storey or double storey, or extension of a 2-storey house to 3-storey which most may not even have proper application with the local town council. The shared common wall is not a load bearing structure so the significance is much lesser with regards to obtaining permits from the town council.
What i think is more important is the neighbour next door who is sharing the common wall. If one wants to modify the common wall, usually to change a half-wall c/w grille to a full-height brick wall, or just like the case of the OP here that is to increase the height of the wall, a common understanding or mutual agreement with the neighbour will be necessary, that is to get consent from the neighbour to have the wall modified. The wall is shared between two units so it can get tricky if the neighbour does not want to have the wall modified. In most cases it is usually down to the cost as the neighbour may not want to share the construction cost.
To the OP - if you increase the height of the wall on one side to match the height of the other wall, your next-door neighbour will then have the same issue of having two different walls with different height. In other words, you resolve your problem of the different height of your wall after you increase the height of the wall, but you will introduce the problem to your neighbour after you have increased the height of the wall. Your neighbour will then have two walls that are different in height. Are you sure your neighbour will allow you to increase the height of the common wall?