Adding high notes is what mostly discussed in articles that talks about how to increase vocal range and ignoring altos and basses. This article will mostly focus on lower voices. By using your chest voice, we'll explore some ways to add some more low notes on your voice.
When you talk, most of us uses the chest voice. In fact, your speaking voice can teach you a lot about your singing voice. You can either help or hinder your singing voice just by the way you use your speaking voice.
Let us explore your speaking voice for starter. Laugh, cry, yawn, sign etc are your various non-speech sounds. Try voicing it out. If you have a piano or pitch pipe available, find the nearest pitch to the sounds you made. Now speak a few monosyllables: uh-huh, mm-hmm, aha. Once again, using the piano or pitch pipe, speak a few monosyllables and match the pitch you produce.
After the non-speech sounds, try a few simple sentences like "I am ___ years old." or "I love to sing". Once again, find the matching pitch. Ideally, the pitch should be the same for speaking as it is for monosyllables or non-speech sounds, but many people try to speak at a lower pitch than is natural for their voice. Doing this is not recommended and is not healthy thing to do.
Continue exploring your voice by speaking monosyllables at various pitch levels on a piano. Without sounding gravelly, try and find the lowest pitch you can speak. The gravelly sound is called "vocal fry" and is not healthy to sustain. Four or five steps above your vocal fry level should be your ideal speaking pitch.
After that, try reading a paragraph or speak some sentences. Experiment with higher speaking pitches to see how high you can go. Along the way, note where your voice is most comfortable and where you start to hear and feel strain.
You will feel vibration or resonance in your chest when you use your 'chest voice'. This is when you produce tones in that pitch range. Place your hand lightly on your upper chest, with your thumb and fingers resting on your collarbones. Do a yawn-slide (exhale on the syllable "hee" or "hoo" while sliding from the top of your range to the bottom). Your hand should feel vibration as you slide down into your chest voice.
You must know that the resonance is happening in your throat and mouth although it may feels like it's occurring in your chest. The vibration you feel is the result of air moving from your lungs and across your vocal folds.
The fifth slide is the simple low-range singing exercise. Use the buzz (that's the puckered lips vibrating as air is expelled) or a syllable such as "vaw", while in the comfortable middle part of your range and sing the starting pitch and slide down five steps. In the key of C major it would be G-C, so-do. The slide should be smooth, not bumpy or creaky. Start each repetition a half-step below the previous one.
You are probably holding some tension if you feel bumpy or creaky sensation when you descend the scale. You stop for a while and do some relaxation exercises for your face and neck. Try doing it again after gently massaging your face and throat. Close your mouth slightly from its starting position as you descend the scale.
Next, sing an octave scale up and back down, again using the buzz or "vaw". Allow your jaw to drop and your mouth to open a bit wider as you go up the scale, then reverse that as you come back down. It may be helpful to imagine your tone on a path leading away from yourself, with low notes nearest and high notes farthest away. Perhaps even move one hand away from your body as you ascend the scale and back to your side as you descend.
The arpeggio is another helpful exercise. Sing do-mi-so-do-so-mi-do on a vowel sound, such as "oo", "ee", or "ah". Start each new arpeggio a half-step lower than the last.
As with any singing technique, adding to your lower range will take time and effort. If you are patient and persistent, you will see positive results.