Learn Turkish: Yakınında kumsal olmayan bir evde sıkılacağız. - We will get bored in a house that does not have a sandy beach near it.

This is a negative existential relative clause. Instead of using var (there is) or yok (there isn't) inside a relative clause, Turkish uses the participle forms of olmak: olan (having) and olmayan (not having). Literally, it translates to "in its vicinity (yakınında) a sandy beach (kumsal) not being (olmayan)". Together, they modify the house to mean "a house that doesn't have a beach near it."

Break down the suffixes in the word yakınında.

The base word here functions as a noun, yakın (vicinity or nearby area). The -ı is the 3rd person possessive suffix linking back to the house ("its vicinity"). Next is the buffer letter -n-, which is always required when attaching a case marker to a 3rd person possessive suffix. Finally, -da is the locative case ("in" or "at"). So, yakınında literally means "in its vicinity".

If sıkılacağız means "we will get bored", why is the house in the locative case (evde)?

The verb sıkılmak can take different cases depending on the context. Here, it describes getting bored while physically being in a location, so it takes the locative case (evde sıkılacağız = we will get bored in the house). If you wanted to say you are getting bored of the house itself, you would use the ablative case (evden sıkılacağız).