This words confuse me. When are
sencilla used? They both mean "simple" but are they used in certain contexts?
Tengo un casa sencilla/simple junto al río.
I wonder if i can use both in this context?
They are synonyms only when you're judging about "complexity" or "dificulty".
The most appropiate in this case would be "casa sencilla", because probably you are refering to one of these meanings:
However, "casa simple" also applies in some contexts. For example:
You'd say "casa simple" to specify it doesn't have complex features not expected in a "normal" house. But if your house is just "not fancy", you say "casa sencilla".