I've decided that I want to improve my current bike instead of buying a new one, I'm wondering what upgrades people would suggest applying to an aluminium hardtail. Do I upgrade wheels, frame, fork, groupset or what first? Bike currently weighs 13.3 kg and I am looking to lose some weight to help me on the climbs. Budget is around £500 for improvements, I would ideally like to cut the weight by about 1 kg, since the itty bitty savings from upgrading pedals, handlebars, seatpost and stem do not amount to much and would end up costing several hundred pounds to drop a maybe 300-400g of weight, which would be less noticeable.

Wheels are the best place to get gains, especially if you can go tubeless.

Already running tubeless, but should I go carbon wheels or just get a lighter alloy. not sure how much wheels currently weigh

I'd still say lighter wheels are the way to go.

After that, I'd start making changes to groupset/drive train if you want to keep the frame.

What bike? Kind of hard to make a recommendation without knowing what you have.

I have CF wheels on my Epic, so I thought about them for my SS. But, the weight difference between good alloy rims and CF rims is not that much, but the price difference is huge. So, go with good alloy rims with great hubs.

It depends...

What wheels, fork, tires and group do you currently have?

Its waste of time to suggest upgrades without knowing what you already have.

Wheels and fork .... if you are talking about upgrading the Frame ... well, that's talking about buying a new bike, pretty much. As others note .... if we don't know what you have and what it weighs it is tough to suggest lighter-weight replacements. If thre wheels and fork arte as light as possible, then a new, lighter frame is your only choice ... assuming the cockpit/drivetrain bits are reasonable. if you are running massively heavy Wal-Mart parts on a good frame ... not sure but I might suggest professional care.

Sorry I couldn't get back to you guys last night, pub calls. I'm running: Fork: Rockshox Recon Gold TK29 120mm Air, Rims: Alex Volar 2.1 32H Front Hub: Formula DC-51 6-bolt disc Rear Hub: Shimano FH-M475 6-bolt Spokes: Stainless PG, SLX shifters, Deore drivetrain, mountain king 2.2 tyres. I can post full tech specs if people want.

How much faith do you have in Chinese carbon ? Willing to go used? How much are you willing to spend? 1kg is a lot of weight to loose on a bike that is already decent. I'm sure you can cut a couple hundred grams off the wheels but that is going to be a pricey option new. I cut 2lbs off my rigid by upgrading to a set of used Stan's Crest wheels for $250us but that bike had insanely heavy wheels on it to begin with. A recon gold is a decent fork so kind of hard to recommend upgrading that for a ton of money but slap a carbon rigid fork on there and you will loose a lot of weight.

Echoing CankerI'd say they look like decent wheels and a decent fork. Industry Nine wheels? New frame? Chinese carbon frame? Your only alternative would seem to be shaving off grams on every component, which as you note, gets expensive fast.

Decide what you can spend. Look at every component, particularly seat, seatpost, stem, bars and see if you can find reasonably priced and reasonably lighter versions ... I guess ... but if you are looking to shave off--more like lop off---a kilogram all at once, that's either a new frame of expensive new (or more reasonably priced used) wheels.

I just picked up a used bike which was really well cared for and upgraded (apparently) more than it was ridden, and it is amazing ... and the price was reasonable for what I got. Craigslist might be your best option for someone selling either parts or a whole bike which you could either strip for parts or just buy a whole new bike.

Be patient and wait for a deal which is just a little shy of too good to be true .... for £500 you can get one set of really good new wheels or maybe a Chinese carbon frame and some good used wheels, but for maybe that or £100--£200 more you might find a used bike in really good shape and move upwards significantly that way.

The Recon Gold is a decent fork but its not super light. You could upgrade and drop almost a quarter pound here.

I am sure there is another half pound to be lost by changing wheels.

If you are currentley running a 2 x 10 or 3 x 10 drive train, changing to 1 x 10 could cut up to a half pound.

You could drop another 100 to 300 grams by changing the tires.

You know what they say- the more you spend the lighter the bike...

Or you could lose 2 lbs (1 kg) through diet and exercise and save a thousand bucks.

Only problem with that is that I am trying to gain weight, I want to put on some muscle.

This is crazy talk!

I just ordered these,
27.5" All Mountain (pair) - 27.5" (650B) - Mountain Wheels - Wheels
Nice sale price of more than half off, my stock rims are 17mm ID and 23mm OD wide. For me the 24mm ID of these new hoops will do fine and as they will spread the tire out a little and allow me to run a narrower tire on the back and save more weight where It counts the most...
I did not want to go to 30mm ID wide rims as I like the feeling of a rounded tire profile, too wide squares them off and makes handling In the wet and or soft stuff a bit unpredictable, been there and done that...

On the 1x system you should first have an honest conversation with yourself, 1X is not for every one.
I did It and love it for the simplicity and quiet operation, also dropped a pound off my bike
And It's silent,,no chain slap, I took off my Lizard skin, It hangs on the wall now

I ride Central Florida, all my climbs are short and punchy so bike weight counts for me. I found my ability to spin the small granny ring of a 3X system was poor. I lost momentum shifting the front rings and often wound up 'Chopping Wood', In too low of a gear.

My Upgrade to the Shimano M8000 1x11 was a big help as It shifts just like the $1,000 systems for only $400 with IMO a small weight penalty at the hub not far out on the wheels so spin up is not hindered by the extra weight of the cassette. This extra weight is at the hub remember ? and the Cassette will last I think three times longer than those $350 cassettes,,It cost $140 I think.

About that honest conversation,,,
Are you strong enough to ride YOUR trails with OUT the granny ring ?
Do you have big hills ?
Lots of climbing ???
Is your bike a heavier model ?
A 1x Is not for everyone or everywhere,,,

Remember this:
You will look cooler pedaling your 3x up a hill than pushing your 1x ~~ TANSTAAFL

TANSTAAFL,, (There Ain't No Such Thing As A Free Lunch)

I weighed my front shifter, two chainrings,and all those bolts, front derailleur and cable with housing, I got 1.1 pounds on the scale...

I drop 28 pounds off me for free,,,

I moved to a carbon handle bar for comfort and eliminated fore arm pump and numb fingers completely,, Carbon cancels out harmonic vibrations that aluminum bars amplify. and I lost 110 grams.

Next I want a lighter saddle and a carbon seat post...

It never ends does it ?