DEAR TIM: My husband and I are having a large (24x18 feet) room addition built. Our house has a full basement. Our builder says the cost to upgrade from a crawl space to a full basement under the room addition is prohibitively expensive. Is this true? How would the two basements be connected? What could be done to waterproof the new foundation? P. A.

DEAR P. A.: I don't know if I necessarily agree with your builder. Yes, there is extra cost involved to turn your proposed crawl space into a basement. However, the added cost to create usable basement space is a worthwhile investment.

From a building standpoint not much has to be done to create this space. Your backhoe is already there to dig for the crawl space. He just needs to stay for an additional 4 - six hours. There is no up charge for the footer. Your foundation walls only need five or so extra feet of height. Add a concrete floor, foundation waterproofing, and an entry opening to this new basement and you are ready to play some ping pong!

I did a quick cost analysis. At current prices (1996) in my city, here is the cost breakdown to achieve the full basement:

• extra excavation $250
• extra foundation height $1,650
• cut opening into old basement $380
• concrete floor $1,050
• foundation waterproofing $480

You get a $150 credit for the floor insulation you would have had for your crawl space. The total cost of the upgrade, less profit and overhead, would be $3,660.

That may seem like a large sum of money to many people. However, let's compare the cost of this usable space on a square foot basis. You only need to spend an additional $8.50 per square foot to get a basement. Compare this to the square foot cost of the room above. I'll bet that your contract price for the room addition is very nearly $45,000. If that is the case, you are paying a little more than $100.00 per square foot for the finished room space. The new basement is a huge value.

Many people who have abandoned cisterns have the capability to create similar spaces. In these cases they simply need to empty the cistern and cut an opening from the existing basement into the cistern. A special contractor uses a large water cooled concrete saw to create the opening. This can be done in my city for less than $400.

Your new foundation should be waterproofed if you intend to use it as a living space. Some companies can spray a modified asphalt that contains flexible compounds. Another process involves spraying a liquid rubber compound. Special panels containing an expansive clay can be used as well. All of these processes are designed to bridge or fill a crack in the foundation which may develop at a later date. They must all be used in conjunction with an excellent drain tile and gravel backfill system. Ordinary asphalt coatings are not waterproof. They will not bridge a crack in your foundation.