A simple way to prototype touch interaction is to extend electrodes from a capacitive touch screen to off-screen areas. With that we aim to develop a toolkit that transforms a user-designed layout into a layout of screen-extension electrodes that realizes touch for rapid prototyping. Nevertheless, this kind of extension cannot detect touch if the physical properties of the electrodes become large. In this work, we decompose the physical design properties of the extension electrode into two factors, the target area, and line bridge, and investigate the limitation of each separately. While revealing some factors, such as area, is extremely limited in term of designing freely, we look into the causes by measuring the capacitive charge on-screen and on-extensions.