I put this together a few weeks ago then forgot about it. We see a lot of these cars and in the past they've just been fairly robotically IDed as 2000-04/05 but there's a few obvious changes that happened during the run that can narrow down the year range. It's useful to keep this as a thread because the knowledge might be lost depending on who's active or looking at a specific time (see the issues with the other police favourite, the Crown Victoria, recently). Also, if any new knowledge is spotted it can be added to this guide.

Original Versions:
Base Impala/9C1:

1. Black sills/rocker panels.
2. Always with black trim on bumpers and side mouldings (unless painted over)
3. Steel wheels on 9C1
4. Plastic covers or multispoke wheels on Base.

Impala LS:

1. Painted sills/rocker panels
2. Front foglights
3. Usually with Five spoke alloy wheels of two different designs
4. Headrests with holes (Standard 2000-03, not always fitted 04-05)

Year Changes:
2000

1. New Impala introduced this year.
2. Two piece bumper trim with body coloured number plate surround. NOTE: some later models have been spotted with this.

2001

1. New bumper trim. One piece with plastic surround to number plate
2. Emergency trunk release added (well we might see that one time)

2002
Nothing found so far

2003
Sports Appearance Package now available on LS trim

Sports Appearance features:

1. Body coloured rear panel
   
2. Body kit with extended shirt, front spoiler etc
3. Slightly different 5 spoke rims to normal 2003 LS.

2004

1. Body coloured bumper trim and side protection strips on Impala LS (Base and 9C1 continue with black plastic trim.)
2. Rear panel is now black rather than tinted red
3. SS trim added to the range.

SS features:

1. Has features of Sports Appearance Package.
2. Specific 17” alloy rims
3. Unique spoiler which is more closed off than the one used on normal models
4. Only available in black.
   

2005

1. Chevrolet bowtie on front is now brass/gold rather than blue
2. Last MY for this generation of Impala
3. SS model now has headrests with holes.
4. New colours offered on the SS (blue, silver, red etc)

Pictures:
2000 bumper trim and 2000-03 rear panel:


2001+ bumper trim and -03 rear panel:


2003 LS with Sports Appearance Package:



2004-05 rear panel:


2004 LS showing the 2004-05 body coloured trim on that trim and the SS:



2005 SS showing the SS specific rear spoiler (Gold rear badging is just an add on):

... and the 2005 gold badge.


And if anyone spots new knowledge or corrections this post can be updated/correctly.

Are these taillights assembly different colour styling between 2001/02 and 2003/04 model years?

2000-03: Dark red panel
2004-05: Black panel
SS/LS Sport: Body coloured panel.

the only change i know is 2004-2005 getting a black panel , 2000-2003 being the same red panel ( except the Sport look that came in 2003 and takes the car's color )

i decided to add a blue SS one as the red one might lead to confusion ( it did for me while my brain recovered the SS data... ) and give the idea they went back to red for 2005



Edit : Same reminder as for the main site... REFRESH the page first, Baube !

Thanks, Sandie

Updating this fellas as we have some information we need to adjust.

@Sandie the LS Sport came in 2002; not 2003 as we have listed: http://www.auto-brochures.com/makes/Chevrolet/Impala/Chevrolet_US%20Impala_2002.pdf - It is listed midway down the brochure and it says new for 2002 LS Sport appearance package.

2000-2002 have optional alloys only for the base (confirmed by the brochures as they aren't listed in 2003): https://www.wheelsandcaps.com/p-20005-aluminum-alloy-wheel-rim-16x65-5083.aspx - note these wheels are not offered on LS models.

2001 is the first year for OnStar rear mounted antenna: http://gtcarlot.com/colors/car/35788187-3.html

2003 was the first year for roof mounted XM radio (little round blob is what I call it) by the brochure: http://www.auto-brochures.com/makes/Chevrolet/Impala/Chevrolet_US%20Impala_2003.pdf

2005 rear change all models are now equipped with a smaller rear mounted antenna (not sharkfin) but the style used prior to that.

All for now

I have one more.

Between the blob and the spike there was another style, similar to the 2005 but stubbier. This change was in January 2004:


The SS used the blob antenna even after this change, and went right to the 2005 spike.

One more detail missed. In 2004 there was 4088 SS Supercharged Indianapolis Motor Speedway Limited Edition and that is a mouthful...

Here is an example of one listed on GTcarlot: http://gtcarlot.com/colors/car/13087889.html
Wikipedia lists it: https://en.wikipedia.org/wiki/Chevrolet_Impala#Eighth_generation_(2000–2005)

@Gamer this edition seems to follow the SS style too.

Edit: adding this newly found document from GM: https://media.chevrolet.com/content/dam/Media/documents/CA/Archives/EN/Vehicles/- chevrolet/2005Impala.html

Adding 2002 & 2003 PDF brochure links for anyone to peruse: http://www.auto-brochures.com/makes/Chevrolet/Impala/Chevrolet_US%20Impala_2002.pdf
http://www.auto-brochures.com/makes/Chevrolet/Impala/Chevrolet_US%20Impala_2003.pdf

More information found to narrow these guys down.
2000-2002 LS alloys were passed over as optional equipment for the base model starting in 2003. I was confused by the LS wheels being on base models without fog lamps and then I read deeper in the brochure and found out they were not optional in 2003 for the base only. LS from 2003-2005 have different alloys.