Hi Jawesh.
This is not 'calculated' in any way shape or form. It has everything to do with the simple laws of supply and demand. Who's out shopping for what? At some times, the difference is 10%... at others, indeed closer to 25%

Also, you need to consider age, location, builder, upgrades, # of bedrooms, etc...

Based on your first sentence, assuming everything else is the same?

Take a 3 bedroom BASIC 1700sqft semi in east Milton, no upgrades, standard lot, might be as low $390k. The same detached in the same area might fetch about $450, that's about a 15% difference.

Take an UPGRADED 1800sqft in derry/scott north, the numbers could hover around the 450k mark. An upgraded loaded detached 1800sqft also in the Vista neighbourhood could clear $550, nearing a 25% spread

All that aside, let's say for 2 weeks straight there are 6 detacheds listed and only 1 semi. Lets say there are 3 buyers out there wanting a semi and 2 buyers wanting a detached... what will happen to the prices for both? Back in the first example, that BASIC semi might end up selling for 405, and a couple detached might drop below 450. Now you're hovering around 10% with one minor temporary shift. Also, WHY are the 6 detached and 1 semi owners selling? Do they HAVE to sell? this will also impact the end prices.

So the question is -- why do you want to know how it's "calculated"? (Because it is not)
If you're educating yourself, for selling you want to know how best to present and market your house, and WHEN to list to achieve a higher selling price. If you're buying, you want to spot the points in the market that allow for the best deals to be had.

Why ask about the trends if you can work the market to create your own?

I guess the more important question is WHY you have asked this question -- then us realtors on this forum can better help you out with the info you need.

Hope this helped, somewhat (?)

Fred

Besides lower price, why would anybody "want" a semi over a detached?

Over a detached?
I don't know, maybe they grew up in a semi and it brings back fond memories of childhood? Maybe in the area somebody is shopping, all detached are over a certain square footage, which they don't need, so a semi is the next best choice?
We both know you asked a question that there really is no answer to. Why don't you take a couple stabs at it hehe

Over an end-unit townhouse -- could be feelings about stigma of "living in a townhouse?"(everybody has different thoughts, wants, and needs) Maybe they don't want to deal with the Access Easements that neighbours have over their property in an end unit townhouse? (which you don't get in a semi)

Semi vs Detached - price is almost always the deciding factor. My post was assuming some buyers were in a lower price threshold than others, which is always the case, and this balance is always changing, month to month, week to week even.