Learn Swedish: Att bygga ett hus kräver stor ansträngning. - To build a house requires great effort.

The entire infinitive clause Att bygga ett hus (To build a house) acts as the subject of this sentence. Because Swedish is a V2 (verb-second) language, the main verb kräver must come immediately after this whole subject block. Adding det would incorrectly push the verb to the third position.

Could I leave out att and just start the sentence with Bygga?

No, you need to keep att. When an action (an infinitive verb) acts as the subject of a sentence, Swedish strictly requires the infinitive marker att (to) at the beginning.

Why is the adjective stor and not stort here?

Adjectives must agree with the gender of the noun they describe. The word for effort is an en-word (en ansträngning), so the adjective takes the basic en-form, stor. If it were an ett-word, it would be stort.

How does the word ansträngning relate to the verb for making an effort?

It comes directly from the verb att anstränga sig (to make an effort / to exert oneself). By dropping the reflexive pronoun sig and adding the suffix -ning, it turns into a noun, very similar to how we add -ing or -tion to English verbs.