OK, let me try if I can get some understanding here on what you are wanting to do.
Firstly, ask yourself, do you really need to use an Arduino to turn a lamp or fan on?
Is there some fancy reason you need to use a computer to light a lamp?
If you are very sure that there is, then I will talk about the lamp for the example, and would like for you to read and accurately confirm each question.
1. To confirm, you have a lamp which is existing and is wired in as a normal house lamp on mains AC system, with switch on the wall?
2. You wish to control this lamp by two means, one is using the existing wall switch, and secondly, by an output from the Arduino?
3. The control is to be independent, whether from the wall switch or the control output of the Arduino?
4. To make the necessary changes to the existing wiring, you will most likely need to have access to the wiring at either the switch or the lamp, can you?
5. Next question is, at the existing wall switch, is there both the un-switched active and the neutral mains wires available?
6. If not, is it available at the location of the lamp?
If the answer is no to both of these questions, then I believe you will have difficulty doing this at all, and you would be advised to look into some form of smart switch possibly with inbuilt remote control circuitry that you can have the Arduino link into. (This would be my preference anyhow).
If you do have un-switched mains available then the next question,
7. Again, to make necessary changes to allow the additional control of the lamp you may need to install a small box with a certain amount of electronic circuitry in the wiring circuit between the switch and the lamp, possibly in the wall near the switch or near the lamp, is this possible, is there room enough to do this?
Only if you can meet the requirements of the above questions and not only for the lamp, but the fan you mention as well, only then is it feasible to discuss some methods on how to do this.
Why loose? Work as OR gate you do not loose the remote function....
'Antkan', you misunderstand niner, he does not mean loose the function of the remote, he means what happens if he misplaces the remote control unit, or the dog eats it. 'Antkan', I know you are keen and excited, especially with your own projects and you are trying in your own way to help, but I notice you are just adding more confusion to the topic.