| C3PROJECTS Ltd | Real estate | 2015 |
| Cafe Fresh (Middleton) Ltd | Hospitality | 2017 |
| Cainburn Media Services Ltd | Administrative & support | 2018 |
| Canal Side Garden Centre Limited | Wholesale & retail | 2023 |
| Capacity for Change Limited | Professional & scientific | 2005 |
| Capri Pizzeria Ltd | Hospitality | 2019 |
| Car Sales (N.E.) Limited | Real estate | 1978 |
| Casa Dune Ltd | Wholesale & retail | 2025 |
| Castle Callcentre Ltd | Administrative & support | 2014 |
| Castle Care Teesdale Limited | Health & social care | 2002 |
| Castle Electrical Solutions Ltd | Construction | 2023 |
| Castle Gardens Ltd | Administrative & support | 2019 |
| Castle Pizza L Ltd | Hospitality | 2025 |
| Castle Safety & Risk Services Ltd | Financial services | 2017 |
| Castle Taxidermy Ltd | Wholesale & retail | 2024 |
| Cavenham Bell Limited | Professional & scientific | 2023 |
| Celerity Investments Limited | Professional & scientific | 2019 |
| Central Hub Group Limited | Construction | 2024 |
| Chatts Leisure Limited | Hospitality | 2015 |
| Chocolate Baroque Ltd | Manufacturing | 2011 |
| Chris Collingwood Building Services Limited | Construction | 2005 |
| CI Atkinson Groundworks Limited | Construction | 2016 |
| CJ Therapeutic Productions Ltd | Health & social care | 2025 |
| Claire Pringle EPA Services Limited | Professional & scientific | 2023 |
| Clinicalm Ltd | Health & social care | 2025 |
| CMM Project Solutions Ltd | Professional & scientific | 2017 |
| Colin Brown (Darlington) Limited | Administrative & support | 2006 |
| Colling Morris Architectural Services Ltd | Professional & scientific | 2015 |
| Come Hill OR High Water CIC | Hospitality | 2022 |
| Concaminchie Music Limited | Arts & recreation | 2011 |
| COR ET Anima Ltd | Health & social care | 2026 |
| Cosmonyx Ltd | Financial services | 2026 |
| Cotherstone Cheese Co. Limited | Manufacturing | 1985 |
| Cotherstone Kitchen Limited | Wholesale & retail | 2013 |
| Coverdale Heritage Properties Limited | Manufacturing | 2008 |
| Coverdale Lettings Ltd | Real estate | 2021 |
| Craig Owen Site Services Limited | Manufacturing | 2019 |
| Crimson Resolutions Limited | Information & communication | 2016 |
| CRM Agri Limited | Agriculture & fishing | 2016 |
| Cross Lanes Organic Farm Limited | Wholesale & retail | 2007 |
| Custom G Limited | Construction | 2003 |
| D & a Coleman Limited | Construction | 2006 |
| D & a Colling Electrical Services Ltd | Construction | 2018 |
| D B L Builders Limited | Construction | 2022 |
| D S & O Carr & SON Limited | Construction | 2005 |
| Daisy Clover Ltd | Wholesale & retail | 2020 |
| Dalesway Yorkshire Limited | Education | 2001 |
| Danger Scrappy Ltd | Information & communication | 2022 |
| Dark Skies Design Limited | Professional & scientific | 2018 |
| Dark Sky Brewery Ltd | Manufacturing | 2021 |
| Darlington LGV Driver Training Ltd | Education | 2025 |
| Daruchini Ltd | Hospitality | 2021 |
| DAS64 Design Limited | Manufacturing | 2019 |
| Davenwill Estates Limited | Financial services | 2025 |
| David Harper Antiques Limited | Wholesale & retail | 2002 |
| David Sowersby Services Limited | Construction | 2020 |
| David Teward Limited | Administrative & support | 2007 |
| Davies Healthcare Limited | Health & social care | 2022 |
| DDH Psychological Solutions Limited | Health & social care | 2011 |
| De-Licious Deli Barnard Castle Limited | Wholesale & retail | 2021 |
| Deedy Consulting Limited | Professional & scientific | 2018 |
| Delilah Films Ltd | Professional & scientific | 2021 |
| Delm Estates Limited | Financial services | 2025 |
| DELTA121 Ltd | Professional & scientific | 2021 |
| Denham Printing Company Limited | Manufacturing | 2009 |
| Dependable Property Services Group Ltd | Manufacturing | 2026 |
| Design by POD Limited | Professional & scientific | 2016 |
| DHP Northern Limited | Professional & scientific | 1998 |
| Dieteren Consulting Limited | Professional & scientific | 2013 |
| Digventures Community CIC | Professional & scientific | 2024 |
| Digventures Limited | Professional & scientific | 2011 |
| District North Limited | Manufacturing | 2018 |
| Docherty Accounting Services Ltd | Professional & scientific | 2026 |
| Docherty Property Investments Limited | Real estate | 2024 |
| Dotheboys Hall (Bowes) Limited | Hospitality | 1986 |
| DR M E Bradshaw's Teesdale Special Flora Research & Conservation Trust | Professional & scientific | 2020 |
| Dream Street Records Ltd | Arts & recreation | 2005 |
| Drew and Associates Ltd | Professional & scientific | 2013 |
| Droneflight Ltd | Professional & scientific | 2012 |
| DRW Building Services Ltd | Construction | 2025 |
| Dummy Ltd | Information & communication | 2023 |
| Durham Dales Enterprise C.I.C. | Real estate | 2013 |
| Durham Development Services Limited | Real estate | 2005 |
| DW Events @ REC Ltd. | Arts & recreation | 2014 |
| Dylan Graham Construction Limited | Construction | 2015 |
| Eala Limited | Financial services | 2020 |
| East Briscoe Limited | Hospitality | 2009 |
| East Layton Stud Limited | Agriculture & fishing | 1981 |
| East Shaws Boarding Kennels & Cattery Limited | Other services | 2003 |
| Eastern Productions Ltd | Manufacturing | 2020 |
| Eastgate Consulting ( Yorkshire ) Limited | Professional & scientific | 2014 |
| EC Property Investors Ltd | Real estate | 2024 |
| Echelon Consultants Ltd | Professional & scientific | 2023 |
| Eco Energ Solutions (Renewables) Limited | Construction | 2014 |
| Eco Energ Solutions Limited | Energy | 2007 |
| Ecomotion Limited | Construction | 2019 |
| Eden Energy Management Ltd | Professional & scientific | 2023 |
| Edje Entertainment Ltd | Hospitality | 2022 |
| EF Paving Contractors Limited | Construction | 2016 |
| Eggleston Hall Limited | Wholesale & retail | 1998 |