House Extension & Loft Conversion Architects in London & Harrow

Many rear extensions fall under Permitted Development — meaning no formal planning application is needed, provided they meet the rules on depth, height, materials and proximity to boundaries. Single-storey rear extensions can often go up to 3–6 metres depending on your property type.

If your plans exceed those limits, or your property is in a conservation area, listed, or has had previous extensions, you'll need a full householder application. We check this during the free consultation and tell you honestly which route applies.

Harrow Council's statutory determination period is 8 weeks for householder applications. In practice, most of ours are decided within 6–10 weeks from submission.

Before submission, allow 3–5 weeks for us to prepare the drawings, forms and supporting documents. If a pre-application enquiry is worthwhile — usually for more complex or borderline schemes — add another 4–6 weeks for the council's response.

Our design fees are fixed and quoted upfront — typically £2,500–£5,500 depending on scope, covering measured survey, concept design, planning drawings and the planning submission.

Build costs are separate and paid directly to your contractor. As a rough London guide, expect £2,400–£3,200 per m² for a rear extension and £45,000–£75,000 for a typical loft conversion. We'll give you a realistic range once we've seen the property.

Planning permission is about whether you're allowed to build it — how it affects neighbours, the street scene and the local area. It's granted by the council's planning department.

Building Regulations are about how it's built — structural safety, fire resistance, insulation, drainage, ventilation. They're separate approvals, checked by a Building Control body, and are required regardless of whether planning permission is needed.

Our core service is design and planning — measured survey, drawings, planning submission and building regulations. Most clients then appoint a contractor directly.

If you'd like us to stay involved through construction, we offer Principal Designer support under CDM 2015 — reviewing the site, coordinating risk and checking the work matches the drawings. It's optional, but recommended for larger or more complex projects.

We're based in Harrow and work across North West London — including Pinner, Stanmore, Northwood, Kenton, Wealdstone, Ruislip, Wembley and Edgware.

We also take on selected projects in surrounding boroughs. If you're outside our usual radius, get in touch — for the right project we'll travel.

CDM 2015 applies to any project with more than one contractor — which includes almost every domestic extension and loft conversion. The role of Principal Designer was introduced to make sure design-stage risks are identified, managed and handed over cleanly to the builder.

For a domestic client, the duties usually fall to the principal contractor if you don't appoint a Principal Designer in writing. We offer this role directly and can take it on from the first conversation.

Book a free consultation — by phone or on site. We'll ask a few questions about the property, what you're hoping to do, and roughly what budget you have in mind. You'll leave the call with a clear view of what's achievable and what it's likely to cost.

If you'd like to proceed, we'll send a fixed-fee proposal in writing within 2 working days. Nothing is billable until you approve it.