明日/家 outdated kanji?

i got a great book for continuing my j studies but i noticed that they wrote out あした instead of 明日 but they had JUST used 昨日 so it didnt make sense why one was spelled out and the other was in kanji, and then i saw them write out うち instead of 家 but i had recently seen that kanji used outside of the book. why did they write these ones out? are they outdated? i thought maybe it was to simplify things but that makes absolutely no sense the way this book has been working so far, and they wrote out things like 郵便局 in kanji so it seems unlikely. so ARE they outdated? it just feels unnecessary that they'd write it in hiragana instead..

Which book?

These kanji are definitely not outdated. They may have just done that to make the reading clear - e.g. 家 is not always うち, 明日 is not always あした.

Just because there is kanji for a word doesn't mean it is always used. There are a lot of words which can be written either in kanji or in kana interchangeably (can be for stylistic reasons or just personal choice).

its just called 'basic japanese' by samuel e martin and eriko sato. i like their way of teaching.

thanks for the answer! later on they used both of those kanji but for 'ie' instead of 'uchi' and then 'asu' instead of 'ashita' so your answer is probably right.