Weather is probably the biggest factor that can delay construction, especially at the beginning of the job, before the roof is complete. For example, roof tiles need at least a few hours to dry, so you don't want to install them if there's a chance it will rain. Once you have the walls and roof finished, bad weather isn't as much of a concern.
The materials you choose and the number of workers on site can also impact your construction timeline. A tile roof takes longer to install than an asphalt one, and if you have four workers, the job goes faster than if you have two. You also need to consider the state of the lot. Demolishing an existing structure adds some time, but more important is the state of the soil. If he discovers bad soil when he breaks ground, your contractor will need to dig and fill in the lot or use a special foundation to account for expansive soil [source: Pacific Crest Inspections].
One holdup that can be out of your contractor's hands is permitting. Sure, if your contractor makes a mistake and the city inspector catches it, that's on him, but sometimes the city gets backed up, and your contractor has to wait for an inspection before work can continue.
So, how can you tell if your contractor is giving you a reasonable time estimate? Rick Bunzel, principal inspector at Pacific Crest Inspections in Anacortes, Wash., says the key is having a good relationship with your general contractor, because he normally has a number of subcontractors who work underneath him and do a lot of the day-to-day work. Each task in home building is interrelated, and your general contractor has to organize that team and all of their timeframes, since each step happens in a sequence. For example, a delay in roughing out the plumbing or electrical system can push back hanging the drywall.
Bunzel recommends using the "reasonable person approach." If something feels like it's taking too long, ask your contractor about it. There could be a good reason, but you won't know unless you ask. It's important to be involved, visit the job site, look and ask questions, and make sure that your contractor is on the job watching what the subs are doing.
"Your relationship with that general contractor is like a marriage" he says. "You may not agree all the time but you're still in it together."