Hello Rezso welcome to the forum
As fasteddy and Silverbear have already mentioned keeping to a three wheeled format will make life a whole lot simpler when it comes to building a road legal lightweight vehicle. Generally so long as it has pedals and it can be moved by muscle power you will be good to go.
Check your local regulations though as things do vary concerning what is allowed and what is not. For example bicycle sidecars are not permitted at all here in New Zealand even though many other countries in the world have no problem with them.
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