I need a lens for our D5000 nikon work camera. The pics we take are pre and post construction, and need as wise a field of view as possible. Most of the rooms are 10 or 12' wide, and I would like to be able to get the whole wall height and length in the pics. I know that 18mm is not wide enough, what should I look into.

You are looking for an ultra-wide angle lens, probably in the 12-14mm range.
Wide angle lenses often have some noticeable distortion affects that are usually easily fixed during post processing.  However, that does bring up the question about how important straight lines are as you may need to get into "perspective correction" (PE) lenses, which add to the price.

Wide angle is not really my thing, but as starting points:
$900   AF-S DX Nikon 10-24mm f/3.5-4.5G ED
$1230 AF-S DX Nikon 12-24mm f/4G IF-ED

Best bet is to upgrade the DX to a FX model and eliminate the 1.5x crop factor, based on your needs.

Sigma has an excellent 10-20mm lens that I own a copy of. Tokina also has a nice wide angle 11-16 f2.8. Both are around $500. The sigma is a slower lens. Pick up a tripod to go along with either lens and you will be good to go.

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As an owner of the Tokina lens, I highly recommend it.  Outstanding lens for the money.



Maybe rent a couple different ones for a day and make sure they fit your needs before purchasing.



 

I have a Nikon refrub'd 10-24mm that I am pretty happy with. You can save quite a ibt of money by buying Nikon refurnished equipment.

The difference is the new Nikon equipment comes in a nicer box and has a 5 year warranty. Whereas the refurb'd stuff comes in cardboard box with a 90 day warranty. You can usually buy an extended warranty for the squimish. If you are a collector don't buy the refurb'd equipment, but if you are user, like me, the refurb'd equipment saves quite a bit of money.

Also buy only from reputable dealers, there are a lot of 'scum bag" companies out there, even if you buy with a CC they don't always side with the consumer.