GSM Builders Ltd are experienced builders in Harrow, working across HA1, HA2 and HA3. Harrow on the Hill, Harrow Weald, Wealdstone, Kenton, Rayners Lane, North Harrow, Pinner and Stanmore borders. We are based in Hayes, roughly 25 minutes away, and regularly run extension, loft and refurbishment projects across the borough.
Building on Harrow’s mix of housing
Harrow has a wider mix of homes than most West London boroughs: 1930s semis in Kenton and Rayners Lane, large detached Edwardian and inter-war houses around Harrow on the Hill and Harrow Weald, Victorian terraces in Wealdstone, and a growing number of flats around Harrow town centre. Each type calls for a different approach, and we have worked on all of them.
Typical projects include double-storey rear extensions on the larger detached houses, hip-to-gable loft conversions on 1930s semis (which can add a genuine 30 m² of floor space), side-return extensions on Wealdstone terraces, and the complete renovation of period houses. Including structural alterations to open up the ground floor.
Planning permission in Harrow
Harrow Council manages planning locally. Harrow on the Hill, Roxeth, Harrow Weald Park and Pinner High Street are among the borough’s conservation areas, where Article 4 directions can remove permitted development rights for extensions and roof alterations. Elsewhere, standard permitted development limits apply. We advise on the right route before you commit, produce the drawings and structural calculations, and submit the application if one is needed. All structural work is inspected and certified by Building Control.
Building services we offer in Harrow
- House extensions, single-storey rear, side-return, wrap-around and double-storey
- Loft conversions. Velux, rear dormer, hip-to-gable and L-shaped
- Full property refurbishment and renovations
- Kitchen fitting and kitchen renovation
- Bathroom renovation and wall tiling
- Roofing, plastering, flooring and painting & decorating
- 24/7 emergency call-outs, plumbing, electrics, glazing and boarding-up
Why choose GSM Builders Ltd in Harrow?
- Genuinely local. Our base at Leven Way, Hayes UB3 is about 25 minutes by road from Harrow, so site visits, quotes and snagging calls are quick and easy.
- One team from start to finish. Design input, structural calculations, Building Control liaison, all trades and decoration under one contract.
- Building Control approved work. Every structural project is signed off with completion certificates you will need when you sell or remortgage.
- Clear written quotes. Itemised pricing, agreed payment stages and no hidden extras.
- Fully insured with public liability cover and a 12-month workmanship warranty.
Examples of work in Harrow
- Two-storey rear extension with new kitchen and master bedroom in Harrow Weald
- Hip-to-gable loft conversion with Juliet balcony in Kenton
- Side-return extension and full ground-floor reconfiguration in Wealdstone
- Kitchen and bathroom renovation of a Rayners Lane semi
What building work costs in Harrow (2026)
These are the 2026 ranges we quote for Harrow (HA1, HA2 and HA3), excluding VAT. They assume a standard specification and reasonable access; the cost calculator gives a figure for your own project, and a free site survey gives you a fixed written price.
| Project | 2026 price (excl. VAT) | Time on site |
|---|---|---|
| Single-storey rear extension (3 m) | £48,000 to £68,000 | 8 to 10 weeks |
| Side-return extension | £45,000 to £65,000 | 8 to 10 weeks |
| Wrap-around extension | £65,000 to £95,000 | 10 to 14 weeks |
| Rear dormer loft conversion | £38,000 to £48,000 | 6 to 8 weeks |
| Hip-to-gable loft with en-suite | £55,000 to £85,000 | 8 to 10 weeks |
| Whole-house refurbishment (3-bed) | £70,000 to £150,000 | 10 to 18 weeks |
| Kitchen fitting (supply & fit) | £12,000 to £30,000 | 2 to 4 weeks |
| Bathroom refit | £8,000 to £20,000 | 2 to 3 weeks |
Drawings, structural calculations and Building Control fees add £2,500 to £4,500. Where planning is needed, Harrow Council charges £548 for a householder application or £249 for prior approval (fees from 1 April 2026). Our planning guide explains which route applies, and the loft conversion comparison covers which loft type suits your roof.
Frequently asked questions
Do I need planning permission for an extension in Harrow?
Harrow has its own planning authority, not Hillingdon or Ealing. Outside conservation areas such as Harrow on the Hill and Pinner Village, single-storey rear extensions up to 3 m (4 m detached) and rear dormer lofts are usually permitted development. Inside them, expect a householder application at £548 and closer scrutiny of roof alterations.
What does a loft conversion cost in Harrow?
£38,000 to £48,000 for a rear dormer and £55,000 to £85,000 for a hip-to-gable with an en-suite, excluding VAT, on the typical 1930s Metroland semis in South Harrow, North Harrow and Rayners Lane.
How quickly can you get to Harrow?
Our yard is on Leven Way in Hayes, about 25 minutes from Harrow, so site visits, deliveries and snagging calls are straightforward. There is no London-wide travel premium in our pricing.
Do you handle Building Control in Harrow?
Yes. Every extension, loft conversion and structural alteration needs Building Regulations approval and a completion certificate regardless of planning. We make the application, book the inspections and hand you the certificate at the end, you will need it when you sell.
Are you insured and do you give a warranty?
Yes, public and employer’s liability insurance, an itemised written quote before anything starts, staged payments against completed work and a 12-month workmanship warranty in writing on every contract.
Nearby areas we also cover
Ruislip · Greenford & Northolt · Hillingdon · North West London · West London
Free quotes in Harrow
Contact us with your address and what you have in mind. We will arrange a site visit and provide a clear written quote at no cost.