10-alarm Waltham fire was set, officials say

For a wood frame building between 4-6 stories, construction typically costs between $260-$280 per square foot. (This is non-union. If it was union, it would be around $320-$350 per SF.)

So say you have a building with average units sizes around 800 square feet, that unit will cost between $200k-$225.

But of course, buildings aren't just the units. You also have to build the common areas (hallways, stairwells, elevators, lobbies). For a very efficient building, common area is about 15-20% of the total square footage. But if this building had any amenities (gym, lounges), it's probably closer to 25%. So now, you have another 200 sf of building at a cost of $50-60k.

So now you're up to around $300k per unit. And now you have to build the parking. Typically, a garage averages out to about 400 SF per spot (once you account for the spot, drive aisles, ramps etc.). If it's above ground, parking costs a little less (about $150 per SF) so each spot costs around $60,000 to build.

In Boston, you can typically get away with a parking ratio of 0.5-1.0 but out here, you're probably at 1.5-2.0, for a parking cost per unit between $90,000-120,000.

This brings you easily into the $400k per unit range, without factoring any of the land cost, plans, engineering, environmental, etc.

Not the only property damaged ... Neighboring businesses were burnt out too.

There are also cleanup and disposal costs to factor in.

They installed a chimney for the backup generator exhaust without adequate clearance. They were testing the backup generator when the chimney heated up in a normal fashion, but since the clearance was extremely low, it heated up everything around it to the point of combustion.