Can the Sandhyavandanam and the japam both performed outside the house - in the garden for instance. I find it more peaceful.
Yes, Sandhyavandanam can be performed both inside the house and outside of it. Some places, which are outside the house, like river banks and temples are considered as the very best places for performing Sandhya.
The following verse from Laghu Satatapa Smriti is quoted in the book "Nitya Karma Puja Prakash".
Griheshu tatsamA sandhyAm goshthe shataguna smritA |
NadyAm shatgunA proktA anantA shiva-sannidhau ||
If performance of SandhyA inside the house gives one unit merit, in the cowpen it gives 100-fold results, in the river banks 1 lakh-fold and infinite results are obtained from doing it in a Shiva temple.
Laghu Satatapa Smriti 114
Here is the PDF of Nitya Karma Puja Prakash. You can find the verse given in the 69th page of the book.
Sandhyavandanam are to be performed 3 times a day facing the direction of the Sun. Example If you are doing in Morning, you should face the Sun (East Direction) and if you are doing at Evening, you should face the West Direction.
As such, there is isn't pre-defined place for it, but logically doing that in open area like Garden/Terrace would help in getting more exposure to sun shine and fresh air.