The verdict in three sentences
A successful migration is measured not on launch day but 8 weeks later: if organic traffic is flat or up, the 301 mapping held. The most underestimated line item is not development but the URL audit and redirect plan, which accounts for 30-40% of the risk. In London in 2026, budget GBP 1,300-3,500 (approx. EUR 1,500-4,000) for the mapping work alone, on top of the technical rebuild.
What breaks SEO during a migration
Traffic losses almost never come from the new design. They come from URLs that change without redirects, lost tags, or a staging robots.txt left in production. Here are the classic causes and their estimated impact (2026 order of magnitude on a 1,200-page B2B site).
| Cause of loss | Frequency | Estimated traffic loss | Cost to fix after the fact |
|---|---|---|---|
| Forgotten 301 redirects | Very common | -15 to -30% | GBP 700-1,800 |
noindex or Disallow left in prod | Common | -40 to -90% | GBP 260-520 |
| Titles/metas not migrated | Common | -5 to -12% | GBP 430-1,300 |
| Lost structured data (JSON-LD) | Medium | -3 to -8% rich results | GBP 350-1,050 |
| Redirect chains (cascading 301s) | Medium | -2 to -6% crawl budget | GBP 260-700 |
| Content truncated at migration | Medium | -8 to -20% long pages | GBP 520-2,200 |
The rule: fixing upstream costs 3 to 5 times less than repairing a ranking drop afterwards, since reclaiming a lost position takes 6 to 12 weeks.
The tools: Screaming Frog vs Ahrefs vs custom crawl
The mapping relies on a full crawl of the old site, cross-referenced with the new URLs. Here are the 2026 options and their cost.
| Tool | 2026 annual cost | Primary use | Limitation |
|---|---|---|---|
| Screaming Frog SEO Spider | ~GBP 200/yr | URL crawl, title/meta extraction, redirect audit | Local, scaling on big sites |
| Ahrefs (Standard) | ~GBP 2,100/yr | Prioritize high-traffic/backlink URLs | Costly for one-off use |
| Sitebulb | ~GBP 360/yr | Visual audit reports for clients | Learning curve |
| Google Search Console | Free | Actually indexed pages, target queries | Data capped at 16 months |
| Custom crawl (script) | Internal time | Specific cases, tailored exports | Maintenance |
The winning combo for a London SME: Screaming Frog for the technical crawl + Search Console to identify the 100 URLs that drive 80% of traffic to protect first.
The 6-step switchover schedule
| Step | Duration | Deliverable | Indicative cost |
|---|---|---|---|
| 1. Audit and export all URLs | 3-5 d | Inventory of 1,200 URLs + traffic | GBP 520-1,050 |
| 2. Old -> new mapping (301) | 5-8 d | Validated mapping table | GBP 780-2,450 |
| 3. Content freeze + staging review | 3-5 d | Crawled preprod site | included in rebuild |
| 4. Acceptance crawl (redirects, 404s) | 2-3 d | Zero broken link report | GBP 350-780 |
| 5. Go-live + sitemap submission | 1 d | Updated sitemap + GSC | GBP 175-430 |
| 6. 8-week monitoring | ongoing | Weekly rankings/traffic tracking | GBP 260-610 |
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The content freeze (no editorial changes during the switchover) avoids mixing migration effects with content effects, and keeps post-launch diagnosis readable.
Mini case study
Mark, digital lead at a B2B software vendor in London, migrates 1,200 URLs from Drupal to Next.js. Organic traffic is 42,000 visits/month, roughly 55 monthly leads worth an average GBP 780 margin per signed client. A 25% traffic drop for 3 months would cost about 13 lost leads, i.e. ~GBP 10,100 in missed margin, before any reclaim effort.
He invests GBP 3,000 in a full 301 mapping and an acceptance crawl. Result at 8 weeks: -3% traffic in week one, back to +2% by week six. A GBP 3,000 insurance policy averted a GBP 10,100 risk: the ratio speaks for itself.
FAQ
How long should 301 redirects stay live? At least 12 months, ideally permanently. Google can take months to consolidate the signal, and external backlinks keep pointing to old URLs for years.
Does a migration always drop traffic? No. A temporary 5-10% dip for 2-4 weeks is common, but a well-migrated site recovers, and even improves if technical performance gets better. A lasting drop signals a redirect problem.
Should you migrate old pages with no traffic? Redirect yes, recreate no. Any indexed URL or one receiving backlinks needs a 301, even without current traffic, to preserve accumulated authority.
Is changing URLs unavoidable in a redesign? No, and keeping the same URL structure is the best scenario: it removes 80% of the SEO risk. Only change URLs if the old structure is genuinely harmful.
How much does post-migration monitoring cost? Budget GBP 260-610 for 8 weeks of weekly tracking (rankings, 404s, traffic, indexing). That is the window in which residual 404s get caught before they cost rankings.
Let's scope your project. Migration to price? Share your current CMS, URL count and target date, and we'll build the redirect plan. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
