My house was built in the early 1950's with a full basement. Somewhere down the line, the previous owners added an extension to the back of the house without extending the basement. What I'd like to do is excavate what's underneath the extension and add on to my pre-existing basement. Any input regarding cost would be appreciated.

The cost per square foot will be very high, and the result would be substandard space. If it's at all possible, adding the equivalent amount of floor area above ground would be less expensive, and would give you much better space quality.

To extend a crawl space to become a basement requires underpinning of all three crawl space walls, and hand excavating the entire space and carrying the soil out almost bucket by bucket. You may have a center girder supported by posts, and the posts would have to be replaced and new footings for them constructed, while the girder is somehow temporarily supported. All of this is very labor-intensive and therefore costly.

There are also some building code issues that need to be addressed for a basement which didn't apply to a crawl space, such as insulation and egress. Depending on your proposed use, there may be others, such as natural light and ventilation.

Without knowing many factors such as the size of the area, your location, your soil conditions, the depth of the excavation, etc., it isn't possible to make any statements about costs, other than to say it won't be cheap, and the result would still only be a basement.

thx for the advice Richard...I didn't realize how much work/money adding an extension would be. The reason I considered doing it in the first place is because my current basement ceilings aren't high enough for the recording studio I'd like to build. I asked a mason who does work on our house once and a while how much it would be to lower the floor and he said very expensive. I figured digging out the crawl space and extending the basement would be just as expensive or less, but with the added bonus of an extra room to justify the costs.

It's basically the same problems and the same work to do either. The crawl space just has more dirt to remove, and needs more concrete to underpin. If you have room, you could almost build a studio above ground for the same money, and you'd be adding value to your property if you designed the room so that it would have another logical use besides a recording studio...like another bedroom, or a den, etc.

That's what I meant by "space quality"...a space that would add value to your house, which a basement wouldn't really do at all.

Very wise advice. Thx for your help.