Difference between пристрóить and подстрóить

This is another one from the test.

The correct phrase is:

К дóму мы реши́ли пристрóить сарáй.

And the wrong answer was 'подстрóить'.

My dictionary isn't very clear but it seems that пристрóить has the sense of 'build, add on' whereas подстрóить has the sense of 'build on, add a wing to a house'. Since your barn would be separate from your house подстрóить is the wrong answer. This seems quite a subtle point. Can anyone explain to be a bit about how I could have told this from the При and Под prefixes?

спасибо

This one is very subtle. I think it can actually be both depending on what you mean. In any case подстроить in my case would mean to build something up close to an existing building, possibly even merging the 2 structures into 1. Expand an existing building if you will, like:

подстроить веранду к дому

Пристроить is to build a separate structure somewhere close to an existing building, in its vicinity. It's a bit strange to keep a barn right close to your house, so it's пристроить then.

iCake - taking you at your word and correcting your English (which is generally flawless) I would just suggest that "in my case" might be better as "in my understanding". Unless, of course, you are referring to your actual barn/house?

Thanks for the correction. I really appreciate it. I think I could say that it's actually my case, but I didn't follow that line of thinking when I was writing that. I had a feeling it was a bit off though.

Hmm... I feel that both of 'при-' and 'под-' can equally mean 'close to' and dictionaries really mention this word as synonyms.
In my perception subtle difference is derived from prefixes. "При-" like in при-делать, при-стыковать, при-совокупить, при-ложить or even при-лететь means 'connected / close to / approach to target'. So it's 'to build smth close/connected to smth'. Normal usage for me is clear "zero-distance". So, I am perceiving 'пристроили сарай' as they share one wall.
'Под-строить' is very rare in usage, imho. 'Под-' is like 'from bottom'/'below'. It can be seen in под-делать, под-ставить, под-прыгнуть, под-держать. For me it's shifting meaning оf '-строить' to 'to be not only close to but also lower than' and 'to do auxilary, probably supporting role'. In this point of view barn is also do not fit to this well.

P.S.
Haha. Looks like I missed the elephant: под-строить is really rare (and a little bit strange) in the meaning of "to build barn". It's first and common meaning is "to build up ground/base for some event". For example "to set up trap". This is there logic of "под-" as "above/bottom" works fine "to prepare ground for...". I bet this meaning was meant by test.

http://domikarkas.ru/wp-content/uplo...u-1024x563.png

Сперва спроектируйте пристройку в программе или на листе бумаге, просчитайте затраты

Каркасная пристройка к дому - увеличиваем жилые метры

http://www.usualhouse.com/catalog/wp...4/08/24738.jpg
Современная пристройка к старому каменному дому





Да, слово подстроить при строительстве редко употребляется. У меня ассоциируется с какой-то небольшой переделкой. Например: подстроить к крыльцу рампу (сделать дом легко доступным для коляски /accessible).