Hello there!
I've recently improved my low-range, "false cord" death metal vocals to the extent that the tonal qualities are sounding good to me and the power is there. However I've noticed that where my clean singing voice is powerful and with a fairly good range of pitch before I begin practising guttural vocals, once I've ran through a few songs that purely use death metal vocals, my singing voice feels quite hoarse and it's a struggle to reach some of the notes I managed to before...
Is this normal for a beginner? if not do any of you vocalists know how I could prevent damaging my voice when I growl?

I find that if I growl for extended periods of time, my falsetto range is virtually inaccessible, and comes out as a hoarse whisper.
My chest voice, however, has never really been affected by harsh vocals, aside that my vocal cords might be slightly more tired, and therefore less powerful. It's likely that the technique itself is putting unnecessary strain on your vocal cords.

I'm sure it's difficult to master both, but it can be done. Guys like Corey Taylor and Dax Riggs have both managed to retain tremendous clean singing voices even after having careers involving a lot of screaming (not exactly growling, but very harsh vocals anyways.) That being said, I can't sing to save my life but I figured it must offer some peace of mind.

a better example to cite would've been Peter Tägtgren since he actively has been doing both in his bands mainly Hypocrisy and Pain.

Dax Riggs can't do harsh anymore and really couldn't much anymore the time they did Agents of Oblivion.

Corey Taylor doesn't do either all that well, his harsh really isn't all that harsh and his singing is pretty terrible.

Peter Tatgren would be the best example yeah. Corey Taylor doesn't do very harsh vocals, it's more like a Phil Anselmo wannabe thing. His clean singing is alright though, IMO.

A friend of mine who does both says this happens to him too. Just don't wear out your voice by trying to force out a "VICTIM OF CHAAAAAAAAANGES" after singing a setlist of Cryptopsy songs.

My voice got noticeably deeper as I did more and more deaths grunts, however I started in high school, maybe my voice wasn't fully developed then. Maybe there is a correlation between the two, but rarely (if ever) do I feel like my voice is hoarse after harsh vocals. My guess is that there is something wrong in your technique or that you are pushing it too hard.

When I first started I could match Geoff Tate of Queensryche note for note, but after touring with speed & death metal vocals for a few years, my speaking voice deepened and I was no longer able to hit the high notes without a lot of effort. After 25 years of metal vox, I can still hit all the rough stuff, but can barely muster a clean high note.

vocal rest days will help. There's a lot of professional singers that do this. Take an entire day and literally don't say a single word the entire day, just rest your vocal cords.

it doesn't really affect my singing voice. I had a raspy growl technique in high school (same thing as Phil whatshisname from Whitechapel) but now my technique is more like Mikael Akerfeldt (before he lost his growl). maybe drink some water every 10 or so minutes.

Doesn't Corey Taylor in particular have problems with his harsh vocals? I don't claim to know, but I believe I've read that he has damaged his voice to the point of being unable to speak at least on one occasion.

My growl technique is quite poor, but it does make my voice harsher, lower and harder to control. At any rate, a healthy lifestyle will definitely help your vocals in every way. Eating a balanced diet, not using much alcohol, getting enough sleep and exercising adequately will contribute to the strength of your voice. Prior to singing, avoid milk products and eat fruits, and most importantly, start drinking water about half an hour before you start, and if you sing for a longer period of time, also make sure you drink enough water afterwards. Many pro singers will tell you that there's no such thing as too much water (within reason, of course). Make sure that the water you drink while singing isn't too cold. Warmish water will be absorbed faster, and it won't irritate your vocal cords.

Regarding technique, just make sure you're completely relaxed while growling. Your throat should ideally feel similar as to when you're singing with a clean tone. High larynx, for one thing, will be detrimental to your voice.

I've heard that you shouldn't practice more than five minutes a day, to avoid lasting damage. It shouldn't be affecting your singing voice at all. Basically, if it hurts, don't do it.

Corey Taylor does a sort of hardcore style yell, he doesn't do death growls. Totally different. I really think Peter Tagtgren is the best example here. If anyone can find info on his singing techniques, I think many of us would be obliged.

To back up what Ilwhyan said, definitely stay hydrated. Also cold water can constrict the vocal cords, which is another reason to drink lukewarm water. Milk will make your throat full of mucous. If you're doing clean singing, apple juice is excellent for keeping the throat crud-free.

Also don't forget the basics. Warm your body up physically, get your blood going a bit, and then thoroughly warm up your voice. There's a thousand warm up videos on youtube you can use, and yes you should use a proper style warm up even before death growls. And you can practice more than 5 minutes per day, just practice correctly, and give your voice rest afterward.

ALL of your power should come from your diaphragm. Take that lesson from George Fisher and basically every opera singer. After doing vocals for a song, I'll often be out of breath like I just exercised, due to my diaphragm and stomach muscles going to town, givin me dat powa! I know I'm not the best vocalist, but I've come a long long way in the past year or so, and I did that by doing my research, practicing, giving my voice plenty of rest, and staying generally healthy. Also I practice multiple styles, from Peter Steele-esque baritone stretches to Alice in Chains harmonies (excellent to help you learn to find the key of the song quickly) to black metal shrieks to lower death growls.

+1. Lately I've been practicing the Dani Filth scream (although Devin Townsend does it way better) and if I do any kind of growling or raspy type vocals beforehand it makes it almost impossible to even get close to it. I never used to warm up, just kind of let out "pissed off" exhales/sighs. Now I'll sing Rooster or something similar before the harsh stuff and not only has it made it easier to get a great tone and control, but I find I can hold growls and screams even longer. Still if I try to do the really high shriek after a set of growling it's twice as hard to get, but it's not as bad as it was before I actually took the time to warm up. I never thought there was any reason to, I could already do what I wanted. Now to carry warming up into my guitar playing and drumming.