Construct was not specifically designed with this sort of grid editing in mind. You may best be served by a program better suited for this
That's not to say it is impossible in Construct - you'd just be coding most of it by hand whereas another 2d RPG-specific program will already have the editor, arrays and algorithms ready for you to work with.
Construct, in its current state, can achieve a similar effect much easier and with more flexibility if you use "layers" instead of "tiles." Using tiled background and sprite objects, you can layer entire set pieces to create your world. For instance, instead of having 20 tiles that comprise a house, you would create each house as its own sprite and place it on your canvas. It's an entirely different way of thinking, but if you don't mind trying something new you'll be surprised at the results.
If what you're going for is a classic-style RPG look and feel though, it will take you quite some time to get it running in Construct. We'll help you if you choose to do this, but don't say we didn't tell you it'd be a long process.