First of all you need to know the range of the viola.

The suggested extended technique notations below are only suggestions. Please feel free to invent your own or just write in the part what you would like the musician to do.
The most common string playing technique, simply meaning to play with the bow.
write arco above the stave for the note or passage to be played.
The player plucks the string of the instrument, rather than bow, to play a note or passage.
write pizz. above the stave for the note or passage to be bowed.
A Bartok Snap is a very loud and percussive pizzicato named after the composer Bela Bartok. Generally they require a bit of preparation and release time, making such a technique hard to pull off in quick succession. The string is pinched between the index finger and the thumb, and is pulled away to then be released and snapped back against the finger board.

Literally translating to 'on the touch', the bow is placed nearer or on the fingerboard. The intensity of sul tasto can be moderated with 'poco' or 'molto' (a little on the fingerboard, or greatly on the fingerboard respectively).
write sul tasto above the stave.
The bow is kept relatively near to the bridge. In doing so it highlights the higher harmonics of each note, producing a much glassier and more nasal tone. The intensity of sul ponticello can be moderated with 'poco' or 'molto' (a little toward the bridge, or greatly toward the bridge respectively).
write sul pont. above the stave.
A double stop is where two notes are played simultaneously, either bowed or plucked. Through double stopping you can create harmonies. Depending on the interval and register, certain double stops are easier than others. Idiomatic stops include anything that includes an open string, 3rds, and 6ths. Keep in mind it is one instrument playing and not two!

Generally considered a trembling tone, string players will rapidly move the bow backwards and forwards to create a continuous and shivering effect. It is possible to control the rate of tremolo with 'poco' or 'molto', for a slow, or fast tremolo respectively.

A glissando is a continuous slide upwards or downwards between notes. There are different kinds of glissando, such as continuous or broken, if the slide has to go up only one string, or across several respectively. Glissandos ideally start with a stopped note as you cannot glide from an open string to another note, but you can glide down to an open string as in the example below.


Literally meaning 'with mute'. String players will normally have a mute they can attach to their instrument. Using a mute generally changes the quality of tone, and lowers its volume. Ideal for quieter music.
write con sord. above the stave.
Each of the open strings has a number of nodes that can be lightly touched that isolates particular overtones of the instrument. Some harmonics speak better than others, and it is always a good idea to consult a player, but generally any of the natural overtones of the open strings will sound strongly. Below are the first 5 overtones of D, to be played on the 3rd string.

It is possible to play other harmonics that are not 'natural' by stopping a note on the fingerboard, and then lightly pressing on a node a 3rd, 4th, or 5th above the stopped note, resulting in different kinds of harmonics. The stopped note is written as normal, and the lightly pressed node is given a diamond head shape.

The player uses the wooden side of the bow to either drag across or strike the strings, producing a wispy and eerie sound, or very percussive effect respectively.
write col legno tratto or col legno battuto above the stave.