Start with the front splitter. It delivers the most visible change per pound spent, works as a standalone upgrade, and sets the visual direction for everything that follows. Add the rear diffuser second, then side skirts to complete the profile. If your budget allows, a full performance package covers all three in one consistent finish for less than buying separately.
Walk into any car meet in the UK and you will see the same thing repeated across every make: one car with a perfect front splitter and nothing else at the rear. Another with a rear diffuser that looks deliberately chosen, yet the front end is stock. And the odd one with side skirts that sit too high because they were fitted without removing the front bumper to check alignment first.
The question of what to buy first is genuinely confusing, because the advice online ranges from vague forum posts to sales copy written by people who have never fitted a single part. Most of it does not account for budget, for how the parts look in isolation versus together, or for the basic reality that some upgrades look unfinished without the others.
This guide is written from the fitting side. After hundreds of BMW, Mercedes and Audi installs at the KITS UK workshop in Featherstone, West Yorkshire, the answer to "what should I do first?" is clear — but the reasoning matters as much as the answer.
Quick Comparison: Splitter vs Diffuser vs Side Skirts
| Upgrade | Visual Impact | Typical Cost | Install Difficulty | Best Standalone? | Priority |
|---|---|---|---|---|---|
| Front Splitter | ★★★★★ | £99–£159 | Moderate | Yes — strong on its own | 1st |
| Rear Diffuser | ★★★★☆ | £99–£139 | Moderate | Yes — better with splitter | 2nd |
| Side Skirts / Extensions | ★★★☆☆ | £99–£139 | Moderate–High | No — best with front + rear | 3rd |
| Full Performance Package | ★★★★★ | £297–£476 | High | Best value — all three parts | Best Value |
Prices correct at time of publication — always verify at kits.style before ordering.
What Does a Front Splitter Do?
The front splitter is the single most impactful exterior upgrade available for most performance cars on UK roads.
Visually, it does two things simultaneously: it lowers the perceived front-end height by cutting a hard horizontal line across the bumper, and it adds visual weight to the nose — making the car look more planted, more deliberate, more aggressive without requiring any other changes to the car.
From a distance of twenty metres, a car with a front splitter and a car without one look like different generations of the same model. That gap in visual effect is difficult to achieve with any other single part at a comparable price.
For BMW owners, the BMW G20 / G21 Front Splitter at £139 is consistently the most popular first upgrade. For F30 and F31 owners, the BMW F30 / F31 Front Splitter at £99 offers the same front-end transformation at a lower entry cost. The BMW F32 / F33 / F36 Front Splitter is also £99 — currently on sale from £139. Mercedes A-Class owners typically start with the W177 A35 Front Aero Package.
The reason most owners start here is straightforward: the front of the car is the first thing anyone sees. Getting that right first means every subsequent upgrade builds on a strong foundation.
Pros and Cons of Front Splitters
Pros
- The strongest single visual change available for the price
- Works as a standalone upgrade — the car looks finished even without a rear diffuser
- Available for most BMW, Mercedes and Audi platforms from under £100
- Gloss black finish works across every exterior colour
- OEM+ appearance — does not look aftermarket when fitted correctly
- Sets the visual direction for all future upgrades
Cons
- Reduces front ground clearance — aggressive kerbs and steep ramps require more care
- Must confirm M Sport vs SE bumper specification before ordering — fitment is bumper-specific
- Pre-LCI and LCI variants of the same model are not interchangeable (G20 is a common example)
- Misaligned splitters are the most common DIY mistake — a 2–3mm gap on one side is visible at distance
- Requires proper clip seating — rushing the installation almost always creates alignment issues
Who should buy this first: Any BMW, Mercedes or Audi owner looking to improve front-end appearance for under £160. It is the correct first purchase for the vast majority of daily-driven performance cars.
What Does a Rear Diffuser Do?
The rear diffuser is the complementary piece to the front splitter — and the second upgrade that makes sense for most builds.
At the rear of the car, the factory bumper on most performance road cars ends fairly flatly. A diffuser adds structured geometry to the lower rear section: fins, channels and depth that give the car a more purposeful, motorsport-referenced appearance. The effect is subtle from a distance but significant once you know it is there — which is why diffusers photograph so well and why a rear diffuser combined with a front splitter makes the car look like a deliberate, considered build rather than a standard car with one modification tacked on.
For BMW owners, the BMW G20 / G21 Rear Diffuser at £139 and the BMW F32 / F33 / F36 Rear Diffuser at £99 are the most common rear upgrades. For Mercedes A-Class owners, the Mercedes A-Class Gloss Black Diffuser with Quad Tips at £289 combines the diffuser and exhaust tips into one rear transformation. The A35 Quad Tips at £199 can also be added independently.
One important note on finish consistency: a rear diffuser from a different supplier than your front splitter will almost always have a slightly different gloss level. Gloss black is not a universal standard. Buying front and rear from the same source removes this risk entirely.
Browse the full BMW body kit collection or the Mercedes-Benz styling collection for matched front and rear options.
Pros and Cons of Rear Diffusers
Pros
- Transforms the rear end from flat and unremarkable to structured and purposeful
- Looks significantly stronger when paired with a front splitter — both ends match
- Available with integrated quad tips on some Mercedes applications — one purchase, two changes
- Gloss black finish consistent with front splitter if sourced from the same supplier
Cons
- Less impactful as a standalone upgrade than a front splitter — the rear is less visible than the front
- Exhaust layout matters — some diffusers are designed for specific tip positions
- F30 and F31 rear designs are not interchangeable — always confirm body style before ordering
- Does not look as finished in isolation as it does with a front splitter to reference against
Who should buy this second: Anyone who already has a front splitter, or who is purchasing both front and rear together as a package. Buying the rear diffuser first — without the front — is possible, but the visual return on investment is lower.
Are Side Skirts Worth Installing?
Side skirts — or side extensions — are the third piece of the exterior upgrade puzzle, and the one most often underestimated in terms of the overall result.
On their own, side skirts have a moderate visual impact. They extend and lower the side profile of the car, reducing the visual gap between the body and the road. For cars that sit slightly high from the factory — many saloons and tourings do — the side extensions make the car look more planted without any suspension changes.
Where side skirts become genuinely transformative is when combined with a front splitter and rear diffuser. The three parts together create a continuous lower aero treatment that wraps the entire car. The eye travels from the splitter at the front, along the side extensions, and finishes at the rear diffuser. That complete visual loop is what makes a performance car look like it has had a proper exterior overhaul rather than a few individual parts added on. Without the side skirts, even a strong front and rear combination can look slightly unfinished from a three-quarter view.
For BMW owners, the BMW G20 / G21 Side Extensions at £139 and the BMW F32 / F33 / F36 Side Extensions at £99 complete the performance package alongside their respective front and rear parts. Browse the complete BMW 3 Series G20 / G21 gloss black collection for all three parts together.
Best Upgrade Order Based on Budget
Under £150 — Start at the front
At this budget, the front splitter is the only correct answer. It delivers the strongest visual change available for the money, works as a standalone upgrade, and sets the direction for everything that follows. Nothing else at under £150 changes how a car looks as much as a well-fitted front splitter in gloss black.
For BMW F30 / F31 owners, the splitter at £99 leaves enough remaining budget for gloss black M Style Grilles at £59 — front aero plus de-chromed front end for around £158 total.
£150–£500 — Front and rear
At this range, the priority is pairing the front splitter with a rear diffuser. The two together transform the car from both ends simultaneously. For F30 / F32 / F33 owners, a splitter and diffuser together total £198 at current sale prices. The BMW F32 / F33 / F36 Front End Package at £217 combines the front splitter, grilles and mirror caps into a single consistent front-end upgrade.
£500+ — Complete exterior transformation
At this budget, a full performance package covering all three parts in one consistent finish is the most efficient use of money. Packages are manufactured in the same production run — no risk of mismatched finishes between parts sourced separately.
| Package | Model | Price | Includes |
|---|---|---|---|
| F31 Touring Full Performance Package | F31 2013–2019 | £297 (was £417) | Splitter, diffuser, side extensions |
| F30 Full Performance Body Kit | F30 2013–2019 | £356 (was £476) | Splitter, diffuser, side extensions |
| F32 Full Performance Package | F32 2013–2020 | £356 (was £476) | Splitter, diffuser, side extensions |
| G21 Touring Full Performance Package | G21 2019–2022 | £417 | Splitter, diffuser, side extensions |
| G20 Pre-LCI Full Performance Package | G20 2019–2022 | £476 | Splitter, diffuser, side extensions |
All prices verified at kits.style at time of publication. Always check before ordering.
Individual Parts vs a Full Body Kit
The financial maths on packages versus individual parts favours packages for anyone intending to eventually buy all three components.
The F30 front splitter, rear diffuser and side extensions bought individually at current prices total slightly more than the full performance package. The package also eliminates the finish consistency risk — every component is manufactured in the same production run, which means the gloss level, the sheen angle and the surface texture are identical across all three parts.
The case for buying individually is genuine when the budget only allows for one part at a time, or when only one or two upgrades are planned and not all three. If the plan is a splitter and nothing else, the individual part is the right call. If the plan is eventually all three, the package is almost always cheaper, more consistent, and requires only one installation session.
Professional Fitting vs DIY
Individual parts — particularly front splitters — are manageable for confident DIY owners with the right tools and patience. Grilles, mirror caps and badges are straightforward regardless of experience level. The risk on aero parts is primarily alignment: a front splitter that is 2–3mm further out on one side than the other is invisible when standing directly in front of the car and immediately obvious from any other angle.
The specific problem with front splitter installation is getting both sides of the clips to seat simultaneously. Most amateur installs that produce alignment issues did not fail at the bumper removal stage — they failed at the clip seating stage, because one side was pushed fully home before the other was checked. This is the step that benefits most from a second pair of hands and from taking the time to stand back and check from distance before finalising.
For a full body kit — splitter, diffuser and side extensions in one session — professional fitting is the right decision for most owners. The combination of parts requires bumper removal at both ends, careful alignment checks at each stage, and an understanding of the correct clip sequence.
Location: Unit 3C Onward Park, Phoenix Avenue, Green Lane Industrial Estate, Featherstone WF7 6EP
Hours: Tuesday–Friday 9am–5pm | Saturday 9am–4pm
Warranty: Lifetime fitting warranty on every installation
Finance: Klarna from £10/month at 0% interest
Final Recommendation: Which One Should You Upgrade First?
Front Splitter
Start here regardless of model or budget. Highest visual return, works standalone, sets the direction for everything that follows.
Rear Diffuser
Add once the front is fitted, or buy front and rear together. Both ends transformed — the car looks complete.
Side Skirts / Extensions
Complete the side profile. Transforms the full-length appearance. Without these, front and rear can look slightly unresolved from a three-quarter view.
Full Performance Package
If the goal is all three, buy the package. Costs less than individual parts, matched finish guaranteed, one installation session.
Frequently Asked Questions
What car modification should I do first?
The front splitter. It delivers the highest visual impact per pound spent of any exterior upgrade available for BMW, Mercedes and Audi. It works as a standalone upgrade and creates the visual foundation for every subsequent modification.
Is a front splitter worth it?
Yes. The visual transformation to the front-end stance and presence is significant at any distance. At £99–£159 for most BMW and Mercedes applications, it is the most cost-effective single exterior change available.
Does a diffuser change performance?
On UK road cars, the performance effect of an aftermarket diffuser is negligible in normal driving conditions. The value of a rear diffuser on a daily-driven road car is entirely visual. The rear transformation is genuine and significant. The performance claim is not meaningful at road speeds.
Are side skirts only cosmetic?
For road use, yes — they reduce the visual gap between the body and the road and complete the full-length exterior appearance alongside a front splitter and rear diffuser. On track, properly designed side skirts contribute to airflow management, but this requires the full underbody treatment to be meaningful.
Is a full body kit cheaper than buying individual parts?
Usually yes, when all three components are intended. KITS UK performance packages combine front splitter, rear diffuser and side extensions in one order, manufactured in the same production run for finish consistency, at a price that is competitive with or lower than buying all three parts individually.
Can I install a splitter myself?
Yes, for confident owners with basic trim tools. The key step is seating the centre clips first, then working outwards simultaneously to avoid misalignment. For a full kit, professional fitting is recommended.
Where can I get a body kit fitted in the UK?
KITS UK offers professional fitting at their West Yorkshire workshop — Unit 3C Onward Park, Phoenix Avenue, Green Lane Industrial Estate, Featherstone WF7 6EP. Open Tuesday–Friday 9am–5pm and Saturday 9am–4pm. All installations include a lifetime fitting warranty. Request a fitting quote or learn more about the fitting service.
Related Guides
All products are aftermarket styling parts. Prices correct at time of publication — always verify at kits.style before ordering.
