What's the largest home improvement project you've undertaken?

[OCGuy81]

What's the largest (either in terms of expenses, time, or both) home improvement project that you've undertaken?

My wife and I are currently working on putting new flooring in our entire downstairs.  When we bought the house, we had a mix of carpet in one room, and good but not great laminate in the kitchen and family room.

We've been taking it all up and putting in slate tile.  What we thought would be something we could knock out over the course of a few weekends has been about a month, though we're nearing the end.

(knock on wood) from our cost standpoint, we've miraculously stayed in our budget. 

[formulanone]

The yearly room-painting project, spread out over a weekend or two, is about as much as I can take. Twice a year, I'll take on some all-day gardening project, along with one spring weekend replanting, et cetera. Anything more than pre-assembled furniture or hanging shelves is going to result in frustration plus hiring professionals.

Anything that involves demolition (while usually great fun) is typically a permanently unfinished work...thus, avoided unless absolutely necessary.

[Truvelo]

The house I bought last year has a garage attached to the side. I'm in the process of going through the planning process to rebuild it as a two storey structure to add an additional bedroom above. Work is expected to start next month.

This is by far the biggest improvement I've making to a house. In the past I've redecorated rooms but nothing on the scale of a major build.

[triplemultiplex]

My folks did a LOT to their place.  They bought an older house and have fixed it up real nice over the last 20 years.
They completely gutted and remodeled the kitchen.
They turned a smallish bedroom into a second bathroom.
They built a garage.
They replaced and expanded a porch, part of which was later incorporated into the dining room.
The house has been re-sided and re-roofed.
They completely gutted the living and dining rooms and relocated the stairs to the second floor.
They had to dig out and replace their sewer lateral to the street.
Once that was done, they added a deck.

The house is barely recognizable from when we first moved in when I was like 10.

[Pete from Boston]

Full house gut and rebuild, interior and exterior.