| Signature Flooring Specialists Ltd | Construction | 2013 |
| Signatureandco Limited | Transport & logistics | 2021 |
| Sika Developments Ltd | Real estate | 2025 |
| Silana Consultancy Limited | Financial services | 2017 |
| Silent HAT Limited | Hospitality | 2001 |
| Silkwood Floorcoverings Ltd | Wholesale & retail | 2001 |
| Silver FOX Developments Limited | Real estate | 2001 |
| Silver Star Developments NE CIC | Real estate | 2025 |
| Silver Street Construction Ltd | Construction | 2014 |
| Silverpoint Design Limited | Professional & scientific | 2000 |
| Simms Media Limited | Professional & scientific | 2025 |
| Simon Bartlett Limited | Construction | 2019 |
| Simon Berry Optometrists Limited | Wholesale & retail | 2008 |
| Simon Grundy Ltd | Professional & scientific | 2018 |
| Simple Communications Limited | Information & communication | 2025 |
| Simpler Digital Limited | Information & communication | 2020 |
| Simply Mugs Limited | Wholesale & retail | 2013 |
| Simply SCM Lets Ltd | Real estate | 2024 |
| Simply Sport Limited | Arts & recreation | 2005 |
| SIMUL144 Ltd | Professional & scientific | 2026 |
| Simulaw Ltd | Information & communication | 2026 |
| Sinkro Ltd | Professional & scientific | 2023 |
| Sinton Surveyors Limited | Professional & scientific | 2023 |
| Sip&Dip Limited | Manufacturing | 2025 |
| Sire Ventures 10 Limited | Real estate | 2024 |
| Sire Ventures 11 Limited | Real estate | 2024 |
| Sire Ventures 12 Limited | Real estate | 2025 |
| Sire Ventures 13 Limited | Real estate | 2025 |
| Sire Ventures 14 Limited | Real estate | 2025 |
| Sire Ventures 15 Limited | Real estate | 2025 |
| Sire Ventures 16 Limited | Real estate | 2026 |
| Sire Ventures 17 Limited | Real estate | 2026 |
| Sire Ventures 18 Limited | Real estate | 2026 |
| Sire Ventures 19 Limited | Real estate | 2026 |
| Sire Ventures 2 Limited | Real estate | 2023 |
| Sire Ventures 20 Limited | Real estate | 2026 |
| Sire Ventures 21 Limited | Real estate | 2026 |
| Sire Ventures 3 Limited | Real estate | 2024 |
| Sire Ventures 4 Limited | Real estate | 2024 |
| Sire Ventures 5 Limited | Real estate | 2024 |
| Sire Ventures 6 Limited | Real estate | 2024 |
| Sire Ventures 7 Limited | Real estate | 2024 |
| Sire Ventures 8 Limited | Real estate | 2024 |
| Sirens Hair and Beauty Limited | Wholesale & retail | 2015 |
| Sirius Demolition Ltd | Water & waste | 2008 |
| Sirius Drilling Ltd | Construction | 2011 |
| Sirius Ecodev (High Meadows II) Limited | Energy | 2023 |
| Sirius Ecodev (High Meadows) Limited | Construction | 2024 |
| Sirius Ecodev (Lambeeth II) Limited | Construction | 2024 |
| Sirius Ecodev (Meyrick) Limited | Construction | 2024 |
| Sirius Engineering Group Ltd | Professional & scientific | 2003 |
| Sirius EOT Limited | Extraterritorial | 2023 |
| Sirius Geotechnical Ltd | Construction | 2003 |
| Sirius Remediation Limited | Water & waste | 2003 |
| Sirius Renewable Energy (Investments) Limited | Energy | 2020 |
| Siriusecodev (Gastard) Ltd | Energy | 2023 |
| Site and Topo Ltd | Construction | 2025 |
| Site Plan Limited | Real estate | 2024 |
| Site Waste Clearance.Com Limited | Construction | 2016 |
| Siwah Limited | Information & communication | 2021 |
| Siwel Investments Ltd | Financial services | 2023 |
| SJ Davison Logistics Ltd | Transport & logistics | 2025 |
| SJ SSC Limited | Education | 2020 |
| SJ Todd Groundworks Ltd | Construction | 2018 |
| Sjaa Consulting Limited | Professional & scientific | 2018 |
| SJB Stores Ltd | Manufacturing | 2026 |
| SJL Solutions Ltd | Professional & scientific | 2025 |
| SJM Durham Limited | Financial services | 2016 |
| SJP Flooring Ltd | Construction | 2020 |
| SK Electrics Ltd | Construction | 2026 |
| Skilled Innovation Durham CIC | Education | 2025 |
| Skinnovationclinicuk Ltd | Other services | 2022 |
| SKL Developments Ltd | Construction | 2016 |
| Sksf Holdings Ltd | Financial services | 2025 |
| Skye Consultancy International Ltd | Professional & scientific | 2026 |
| Skylark Consultancy Ltd | Professional & scientific | 2023 |
| Skylark Controls and Engineering Limited | Manufacturing | 2014 |
| Skylark Leadership Ltd | Administrative & support | 2026 |
| Skyscraper Trade Ltd | Wholesale & retail | 2025 |
| SL Hunter Construction Ltd | Construction | 2020 |
| SL72 Solutions Ltd | Information & communication | 2016 |
| Slabman Ltd | Wholesale & retail | 2025 |
| Slashrosa Limited | Manufacturing | 2025 |
| Sldcnr Ltd | Wholesale & retail | 2025 |
| Sleep Reset Ltd | Professional & scientific | 2024 |
| Sleetburn Electrical Services Ltd | Construction | 2024 |
| Slffilms Limited | Information & communication | 2012 |
| SLM Couriers Ltd | Transport & logistics | 2022 |
| Sloanes Media Group Limited | Information & communication | 2022 |
| SLR Holdings (NE) Ltd | Hospitality | 2026 |
| SLS Assessments Ltd | Health & social care | 2023 |
| SLS Home Improvements Ltd | Construction | 2022 |
| SM (Durham) Property Ltd | Real estate | 2025 |
| SM Autos NE Ltd | Wholesale & retail | 2024 |
| SM Event Solutions Ltd | Administrative & support | 2022 |
| Smaki Foods Ltd | Hospitality | 2025 |
| Smart IT Web Development Limited | Information & communication | 2023 |
| Smart Ways Heating & Plumbing Ltd | Construction | 2020 |
| Smartbiofilm Limited | Professional & scientific | 2021 |
| Smashburg Ltd | Hospitality | 2025 |