So if I'm understanding your argument, by only adding 2 and not 4 parking spaces, there is the possibility that a future owner may decide to buy the house and at some point decide to use up more street parking due to the lack of available parking, thus hurting the neighborhood.
If this is correct, then I see a few problems.
First, the actions of the home owner at the time the street parking is being used are the one causing the harm. If I put up a bill board criticizing a politician, and someone upon seeing it decides to engage in politically motivated violence, I am not responsible for that harm as there was a free willed human intervening between the actions and the harm. There is a legal term for this but I forget it for now.
Second, the idea that using parking spaces counts as harm. If so, wouldn't this mean that all use of parking spaces is causing harm? And if the argument is that use under the rules doesn't constitute harm, then that should also apply in this case and the home owner is only causing harm if they are not following the rules of the public parking spaces, something which there should already be a remedy for at the time the rule is broken.
Second part two, if we continue with the idea that use of public parking spaces is a harm, I think we would see a number of negative consequences that are not agreeable with our modern sensibilities.
Third, wouldn't this mean that building a parking lot with only 4 spaces can also cause harm as it is possible for a house to have more than two cars. How does one determine what level of harm is justified enough to be banned, as I find the level of harm between 2 and 4 parking spaces to be so small that I doubt it is actually the exact level of harm that justifies banning something.
Third, this is assuming a new variable which wasn't in the original argument. Thus if this is a road without public parking, this example wouldn't constitute harm. So even if we accept the argument as justified, it only applies to some cases and thus the overall law is still not justified in other cases. While we don't normally formalize a sentence such as 'this law is justified' any further, I think if we were then the formalization would be far closer to 'this law is justified in all cases' 'this law is justified in some cases', as in the latter case we would say the law is unjustified in some cases and generally considered it an unjust law.