As stated in other answers, many architectural shooters will use a Tilt-Shift lens in order to get straight lines done in camera. Or, for a crop sensor camera, a 10-20mm zoom gives you super wide angles to wide angle in available focal length.

But, if we limit things to just your 18-55 and the 24mm prime...

Is it as effective as using a 24mm true wide angle lens for indoor real estate photography?

First up, 24mm is 24mm. Using your 18-55 set to 24mm will give the same field of view as using the 24mm lens. There exist differences between the two lenses, of course, like available aperture to use, sharpness, chromatic aberration, etc.

But, if we just look at the composed photo, they'd be the same.

The only reason that you should consider a lens like a 24mm prime, one that duplicates a focal length already in your bag, is if those other attributes are needed (more open aperture, sharper, less CA, etc). Given that, for architecture, you can use a tripod...many of the benefits of the 24mm prime will be negated.

But, let's say that it did have some benefit. The next question is do you even use that focal length?

Go back though your images and check the EXIF data - which focal lengths do you use the most? If you are at the 18mm end most often, and also finding that you wish you could shoot wider or are doing panos/stitching images...then there would be no reason at all to get a 24mm.

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A note on lens acquisition...if possible, try before you buy by renting or borrowing.

A general path is to expand the focal lengths that you can capture instead of simply upgrading or duplicating focal lengths with primes.

For example, a common kit is 18-55mm, 55-250mm, and a 50mm f/1.8 prime. Why was the prime thrown in when you already have two lenses around that 50mm range? Because of the much more open aperture; in low light, it'll be the only lens you have available for most things.

Your kit has an 18-55 and you do real estate/architecture photography...it would make more sense to get a 10-20mm lens to expand your focal length range than to get anything in the 18 to 55 space that happens to be just a little sharper, just a little less CA, etc.